Native group milestones

53 files changed, 694 insertions, 357 deletions

diff --git a/app/controllers/groups/milestones_controller.rb b/app/controllers/groups/milestones_controller.rb
index 97eed419595..f33989cddff 100644
--- a/app/controllers/groups/milestones_controller.rb
+++ b/app/controllers/groups/milestones_controller.rb
@@ -2,13 +2,13 @@ class Groups::MilestonesController < Groups::ApplicationController
   include MilestoneActions
 
   before_action :group_projects
-  before_action :milestone, only: [:show, :update, :merge_requests, :participants, :labels]
-  before_action :authorize_admin_milestones!, only: [:new, :create, :update]
+  before_action :milestone, only: [:edit, :show, :update, :merge_requests, :participants, :labels]
+  before_action :authorize_admin_milestones!, only: [:edit, :new, :create, :update]
 
   def index
     respond_to do |format|
       format.html do
-        @milestone_states = GlobalMilestone.states_count(@projects, group)
+        @milestone_states = GlobalMilestone.states_count(group_projects, group)
         @milestones = Kaminari.paginate_array(milestones).page(params[:page])
       end
       format.json do
@@ -18,7 +18,7 @@ class Groups::MilestonesController < Groups::ApplicationController
   end
 
   def new
-    @milestone = GroupMilestone.new
+    @milestone = Milestone.new
   end
 
   def create
@@ -35,12 +35,21 @@ class Groups::MilestonesController < Groups::ApplicationController
   def show
   end
 
+  def edit
+    render_404 if @milestone.is_legacy_group_milestone?
+  end
+
   def update
-    @milestone.milestones.each do |milestone|
-      Milestones::UpdateService.new(milestone.project, current_user, milestone_params).execute(milestone)
+    milestones = @milestone.milestones if @milestone.is_legacy_group_milestone?
+    # Keep this compatible with legacy group milestones where we have to update
+    # all projects milestones at once.
+    milestones ||= Array(@milestone)
+
+    milestones.each do |milestone|
+      Milestones::UpdateService.new(milestone.parent, current_user, milestone_params).execute(milestone)
     end
 
-    redirect_back_or_default(default: milestone_path(@milestone.title))
+    redirect_to milestone_path(@milestone.title)
   end
 
   private
@@ -50,23 +59,23 @@ class Groups::MilestonesController < Groups::ApplicationController
   end
 
   def milestone_params
-    params.require(:group_milestone).permit(:title, :description, :start_date, :due_date, :state_event)
+    params.require(:milestone).permit(:title, :description, :start_date, :due_date, :state_event)
   end
 
   def milestone_path(title)
-    group_milestone_path(@group, title.to_slug.to_s, title: title)
+    group_milestone_path(group, title.to_slug.to_s, title: title)
   end
 
   def milestones
-    @group_milestones = GroupMilestonesFinder.new(group, params).execute
-    @project_milestones = ProjectMilestonesFinder.new(@projects, params).execute
+    milestones = MilestonesFinder.new(groups: group, params: params).execute
+    legacy_milestones = GroupMilestone.build_collection(group, group_projects, params) || []
 
-    @group_milestones + @project_milestones
+    milestones + legacy_milestones
   end
 
   def milestone
     @milestone =
-      @group.milestones.find_by_title(params[:title]) || GroupMilestone.build(@group, @projects, params[:title])
+      group.milestones.find_by_title(params[:title]) || GroupMilestone.build(group, group_projects, params[:title])
 
     render_404 unless @milestone
   end
diff --git a/app/controllers/projects/milestones_controller.rb b/app/controllers/projects/milestones_controller.rb
index ea9891697b7..884bc8e6fea 100644
--- a/app/controllers/projects/milestones_controller.rb
+++ b/app/controllers/projects/milestones_controller.rb
@@ -13,20 +13,15 @@ class Projects::MilestonesController < Projects::ApplicationController
   respond_to :html
 
   def index
-    @milestones =
-      case params[:state]
-      when 'all' then @project.milestones
-      when 'closed' then @project.milestones.closed
-      else @project.milestones.active
-      end
-
     @sort = params[:sort] || 'due_date_asc'
-    @milestones = @milestones.sort(@sort)
+    @milestones = milestones.sort(@sort)
 
     respond_to do |format|
       format.html do
         @project_namespace = @project.namespace.becomes(Namespace)
-        @milestones = @milestones.includes(:project)
+        # Shows only projects milestones on list
+        # although  we need to show them in the dropdown.
+        @milestones = @milestones.where(group: nil).includes(:project)
         @milestones = @milestones.page(params[:page])
       end
       format.json do
@@ -87,6 +82,16 @@ class Projects::MilestonesController < Projects::ApplicationController
 
   protected
 
+  def milestones
+    @milestones ||= begin
+      if @project.group && can?(current_user, :read_group, @project.group)
+        group = @project.group
+      end
+
+      MilestonesFinder.new(projects: @project, groups: group, params: params).execute
+    end
+  end
+
   def milestone
     @milestone ||= @project.milestones.find_by!(iid: params[:id])
   end
diff --git a/app/finders/group_milestones_finder.rb b/app/finders/group_milestones_finder.rb
deleted file mode 100644
index d5bf810c41b..00000000000
--- a/app/finders/group_milestones_finder.rb
+++ /dev/null
@@ -1,12 +0,0 @@
-class GroupMilestonesFinder
-  attr_reader :group, :params
-
-  def initialize(group, params)
-    @group = group
-    @params = params
-  end
-
-  def execute
-    GroupMilestone.filter_by_state(group.milestones, params[:state])
-  end
-end
diff --git a/app/finders/issuable_finder.rb b/app/finders/issuable_finder.rb
index 7bc2117f61e..8d40f40d33e 100644
--- a/app/finders/issuable_finder.rb
+++ b/app/finders/issuable_finder.rb
@@ -330,12 +330,7 @@ class IssuableFinder
       elsif filter_by_started_milestone?
         items = items.left_joins_milestones.where('milestones.start_date <= NOW()')
       else
-        items = items.with_milestone(params[:milestone_title])
-        items_projects = projects(items)
-
-        if items_projects
-          items = items.where(milestones: { project_id: items_projects })
-        end
+        items = filter_by_project_and_group_milestones(items)
       end
     end
 
@@ -432,4 +427,17 @@ class IssuableFinder
 
     ['issuables_count', klass.to_ability_name, opts.sort]
   end
+
+  def filter_by_project_and_group_milestones(items)
+    items = items.with_milestone(params[:milestone_title])
+
+    items_projects = projects(items)
+
+    if items_projects
+      items_group = project? ? project.group : Group.find_by_id(items_projects.group_ids)
+
+      milestones = MilestonesFinder.new(projects: items_projects, groups: items_group, params: { state: 'all' }).execute
+      items.where(milestone: milestones)
+    end
+  end
 end
diff --git a/app/finders/milestones_finder.rb b/app/finders/milestones_finder.rb
new file mode 100644
index 00000000000..7f18e889ad3
--- /dev/null
+++ b/app/finders/milestones_finder.rb
@@ -0,0 +1,29 @@
+# Searchs for group milestones and project milestones.
+#
+# Parameters
+# projects: array of projects or single project
+# groups: array of groups or single group
+# params: Search params
+
+class MilestonesFinder
+  attr_reader :projects, :groups, :params
+
+  def initialize(projects: nil, groups: nil, params: {})
+    @projects = Array(projects)
+    @groups = Array(groups)
+    @params = params
+  end
+
+  def execute
+    conditions = []
+    table = Milestone.arel_table
+    project_ids = projects&.map(&:id)
+    group_ids = groups&.map(&:id)
+
+    conditions << table[:group_id].in(group_ids) if group_ids
+    conditions << table[:project_id].in(project_ids) if project_ids
+
+    milestones = Milestone.where(conditions.reduce(:or)).reorder("due_date ASC")
+    Milestone.filter_by_state(milestones, params[:state])
+  end
+end
diff --git a/app/finders/project_milestones_finder.rb b/app/finders/project_milestones_finder.rb
deleted file mode 100644
index a9ffbc9de55..00000000000
--- a/app/finders/project_milestones_finder.rb
+++ /dev/null
@@ -1,14 +0,0 @@
-class ProjectMilestonesFinder
-  attr_reader :projects, :params
-
-  def initialize(projects, params)
-    @projects = projects
-    @params = params
-  end
-
-  def execute
-    milestones = Milestone.of_projects(projects)
-
-    Milestone.filter_by_state(milestones, params[:state])
-  end
-end
diff --git a/app/helpers/milestones_helper.rb b/app/helpers/milestones_helper.rb
index f346e20e807..ae70172973b 100644
--- a/app/helpers/milestones_helper.rb
+++ b/app/helpers/milestones_helper.rb
@@ -54,8 +54,10 @@ module MilestonesHelper
   def milestone_class_for_state(param, check, match_blank_param = false)
     if match_blank_param
       'active' if param.blank? || param == check
+    elsif param == check
+      'active'
     else
-      'active' if param == check
+      check
     end
   end
 
diff --git a/app/models/concerns/internal_id.rb b/app/models/concerns/internal_id.rb
index 5382dde6765..67a0adfcd56 100644
--- a/app/models/concerns/internal_id.rb
+++ b/app/models/concerns/internal_id.rb
@@ -8,7 +8,8 @@ module InternalId
 
   def set_iid
     if iid.blank?
-      records = project.send(self.class.name.tableize)
+      parent = project || group
+      records = parent.send(self.class.name.tableize)
       records = records.with_deleted if self.paranoid?
       max_iid = records.maximum(:iid)
 
diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index cc39fe6eb8e..c4fcab17f9f 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -30,7 +30,6 @@ module Issuable
     belongs_to :updated_by, class_name: "User"
     belongs_to :last_edited_by, class_name: 'User'
     belongs_to :milestone
-    belongs_to :group_milestone
 
     has_many :notes, as: :noteable, inverse_of: :noteable, dependent: :destroy do
       def authors_loaded?
diff --git a/app/models/concerns/milestoneish.rb b/app/models/concerns/milestoneish.rb
index 60aeab38118..6475390edae 100644
--- a/app/models/concerns/milestoneish.rb
+++ b/app/models/concerns/milestoneish.rb
@@ -40,7 +40,7 @@ module Milestoneish
   def issues_visible_to_user(user)
     memoize_per_user(user, :issues_visible_to_user) do
       IssuesFinder.new(user, issues_finder_params)
-        .execute.preload(:assignees).where(issues_finder_conditions)
+        .execute.preload(:assignees).where(milestone_id: milestoneish_ids)
     end
   end
 
@@ -71,23 +71,19 @@ module Milestoneish
   end
 
   def is_group_milestone?
-    respond_to?(:group_id)
+    false
   end
 
   def is_project_milestone?
-    respond_to?(:project_id)
+    false
   end
 
-  def title=(value)
-    write_attribute(:title, sanitize_title(value)) if value.present?
+  def is_legacy_group_milestone?
+    false
   end
 
   private
 
-  def sanitize_title(value)
-    CGI.unescape_html(Sanitize.clean(value.to_s))
-  end
-
   def count_issues_by_state(user)
     memoize_per_user(user, :count_issues_by_state) do
       issues_visible_to_user(user).reorder(nil).group(:state).count
@@ -105,8 +101,4 @@ module Milestoneish
   def issues_finder_params
     {}
   end
-
-  def issues_finder_conditions
-    { milestone_id: milestoneish_ids }
-  end
 end
diff --git a/app/models/concerns/shared_milestone_properties.rb b/app/models/concerns/shared_milestone_properties.rb
deleted file mode 100644
index 77a8da9fd93..00000000000
--- a/app/models/concerns/shared_milestone_properties.rb
+++ /dev/null
@@ -1,74 +0,0 @@
-module SharedMilestoneProperties
-  extend ActiveSupport::Concern
-
-  include StripAttribute
-  include CacheMarkdownField
-
-  included do
-    has_many :issues
-    has_many :merge_requests
-    has_many :labels, -> { distinct.reorder('labels.title') },  through: :issues
-
-    validate :uniqueness_of_title, if: :title_changed?
-
-    scope :active, -> { with_state(:active) }
-    scope :closed, -> { with_state(:closed) }
-
-    validate :start_date_should_be_less_than_due_date, if: proc { |m| m.start_date.present? && m.due_date.present? }
-    strip_attributes :title
-    alias_attribute :name, :title
-
-    state_machine :state, initial: :active do
-      event :close do
-        transition active: :closed
-      end
-
-      event :activate do
-        transition closed: :active
-      end
-
-      state :closed
-
-      state :active
-    end
-
-    alias_attribute :name, :title
-
-    cache_markdown_field :title, pipeline: :single_line
-    cache_markdown_field :description
-  end
-
-  module ClassMethods
-    def filter_by_state(milestones, state)
-      case state
-      when 'closed' then milestones.closed
-      when 'all' then milestones
-      else milestones.active
-      end
-    end
-  end
-
-  def start_date_should_be_less_than_due_date
-    if due_date <= start_date
-      errors.add(:start_date, "Can't be greater than due date")
-    end
-  end
-
-  def safe_title
-    title.to_slug.normalize.to_s
-  end
-
-  private
-
-  # Milestone titles must be unique across project milestones and group milestones
-  def uniqueness_of_title(group, project = nil)
-    if group
-      title_exists = group.milestones.find_by_title(title).present?
-      title_exists ||= Milestone.where(project: group.projects).find_by_title(title).present?
-    elsif project
-      title_exists = project.milestones.find_by_title(title).present?
-    end
-
-    errors.add(:title, "Must be unique across project milestones and group milestones.") if title_exists
-  end
-end
diff --git a/app/models/global_milestone.rb b/app/models/global_milestone.rb
index 8ef4546ee24..d1d90ab638c 100644
--- a/app/models/global_milestone.rb
+++ b/app/models/global_milestone.rb
@@ -2,7 +2,7 @@ class GlobalMilestone
   include Milestoneish
 
   EPOCH = DateTime.parse('1970-01-01')
-  STATE_COUNT_HASH = { opened: 0, closed: 0, all: 0 }
+  STATE_COUNT_HASH = { opened: 0, closed: 0, all: 0 }.freeze
 
   attr_accessor :title, :milestones
   alias_attribute :name, :title
@@ -12,7 +12,7 @@ class GlobalMilestone
   end
 
   def self.build_collection(projects, params)
-    child_milestones = ProjectMilestonesFinder.new(projects, params).execute
+    child_milestones = MilestonesFinder.new(projects: projects, params: params).execute
 
     milestones = child_milestones.select(:id, :title).group_by(&:title).map do |title, grouped|
       milestones_relation = Milestone.where(id: grouped.map(&:id))
@@ -30,18 +30,18 @@ class GlobalMilestone
   end
 
   def self.states_count(projects, group = nil)
-    projects_milestones_count = legacy_group_milestone_states_count(projects)
+    legacy_group_milestones_count = legacy_group_milestone_states_count(projects)
     group_milestones_count = group_milestones_states_count(group)
 
-    projects_milestones_count.merge(group_milestones_count) do |k, project_milestones_count, group_milestones_count|
-      project_milestones_count + group_milestones_count
+    legacy_group_milestones_count.merge(group_milestones_count) do |k, legacy_group_milestones_count, group_milestones_count|
+      legacy_group_milestones_count + group_milestones_count
     end
   end
 
   def self.group_milestones_states_count(group)
     return STATE_COUNT_HASH unless group
 
-    relation = GroupMilestonesFinder.new(group, state: 'all').execute
+    relation = MilestonesFinder.new(groups: group, params: { state: 'all' }).execute
     grouped_by_state = relation.reorder(nil).group(:state).count
 
     {
@@ -51,10 +51,11 @@ class GlobalMilestone
     }
   end
 
+  # Counts the legacy group milestones which must be grouped by title
   def self.legacy_group_milestone_states_count(projects)
     return STATE_COUNT_HASH unless projects
 
-    relation = ProjectMilestonesFinder.new(projects, state: 'all').execute
+    relation = MilestonesFinder.new(projects: projects, params: { state: 'all' }).execute
     project_milestones_by_state_and_title = relation.reorder(nil).group(:state, :title).count
 
     opened = count_by_state(project_milestones_by_state_and_title, 'active')
diff --git a/app/models/group.rb b/app/models/group.rb
index 93e381712bf..78566285159 100644
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -18,7 +18,7 @@ class Group < Namespace
 
   has_many :requesters, -> { where.not(requested_at: nil) }, dependent: :destroy, as: :source, class_name: 'GroupMember'
 
-  has_many :milestones, class_name: 'GroupMilestone'
+  has_many :milestones
   has_many :project_group_links, dependent: :destroy
   has_many :shared_projects, through: :project_group_links, source: :project
   has_many :notification_settings, dependent: :destroy, as: :source
diff --git a/app/models/group_milestone.rb b/app/models/group_milestone.rb
index c518fe430bd..65249bd7bfc 100644
--- a/app/models/group_milestone.rb
+++ b/app/models/group_milestone.rb
@@ -1,35 +1,23 @@
-class GroupMilestone < ActiveRecord::Base
-  include SharedMilestoneProperties
-  include Milestoneish
-  include CacheMarkdownField
+class GroupMilestone < GlobalMilestone
+  attr_accessor :group
 
-  belongs_to :group
-
-  class << self
-    # Build legacy group milestone which consists on all project milestones
-    # with the same title.
-    def build(group, projects, title)
-      GlobalMilestone.build(projects, title).tap do |milestone|
-        milestone&.group = group
-      end
+  def self.build_collection(group, projects, params)
+    super(projects, params).each do |milestone|
+      milestone.group = group
     end
   end
 
-  def milestoneish_ids
-    id
-  end
-
-  def participants
-    User.joins(assigned_issues: :group_milestone).where("group_milestones.id = ?", id).uniq
+  def self.build(group, projects, title)
+    super(projects, title).tap do |milestone|
+      milestone&.group = group
+    end
   end
 
-  private
-
-  def uniqueness_of_title
-    super(group)
+  def issues_finder_params
+    { group_id: group.id }
   end
 
-  def issues_finder_conditions
-    { group_milestone_id: milestoneish_ids }
+  def is_legacy_group_milestone?
+    true
   end
 end
diff --git a/app/models/milestone.rb b/app/models/milestone.rb
index 601346be318..7936fd8a9f2 100644
--- a/app/models/milestone.rb
+++ b/app/models/milestone.rb
@@ -7,17 +7,53 @@ class Milestone < ActiveRecord::Base
   Upcoming = MilestoneStruct.new('Upcoming', '#upcoming', -2)
   Started = MilestoneStruct.new('Started', '#started', -3)
 
-  include SharedMilestoneProperties
   include InternalId
   include Sortable
   include Referable
   include Milestoneish
+  include StripAttribute
+  include CacheMarkdownField
+
+  cache_markdown_field :title, pipeline: :single_line
+  cache_markdown_field :description
 
   belongs_to :project
+  belongs_to :group
 
+  has_many :issues
+  has_many :labels, -> { distinct.reorder('labels.title') },  through: :issues
+  has_many :merge_requests
   has_many :events, as: :target, dependent: :destroy
 
   scope :of_projects, ->(ids) { where(project_id: ids) }
+  scope :of_groups, ->(ids) { where(group_id: ids) }
+  scope :active, -> { with_state(:active) }
+  scope :closed, -> { with_state(:closed) }
+
+  validates :group, presence: true, unless: :project
+  validates :project, presence: true, unless: :group
+
+  validate :uniqueness_of_title, if: :title_changed?
+  validate :milestone_type_check
+  validate :start_date_should_be_less_than_due_date, if: proc { |m| m.start_date.present? && m.due_date.present? }
+
+  strip_attributes :title
+
+  state_machine :state, initial: :active do
+    event :close do
+      transition active: :closed
+    end
+
+    event :activate do
+      transition closed: :active
+    end
+
+    state :closed
+
+    state :active
+  end
+
+  alias_attribute :name, :title
 
   class << self
     # Searches for milestones matching the given query.
@@ -33,6 +69,14 @@ class Milestone < ActiveRecord::Base
 
       where(t[:title].matches(pattern).or(t[:description].matches(pattern)))
     end
+
+    def filter_by_state(milestones, state)
+      case state
+      when 'closed' then milestones.closed
+      when 'all' then milestones
+      else milestones.active
+      end
+    end
   end
 
   def self.reference_prefix
@@ -111,13 +155,21 @@ class Milestone < ActiveRecord::Base
     format_reference = milestone_format_reference(format)
     reference = "#{self.class.reference_prefix}#{format_reference}"
 
-    "#{project.to_reference(from_project, full: full)}#{reference}"
+    if project
+      "#{project.to_reference(from_project, full: full)}#{reference}"
+    elsif group
+      "#{group.to_reference}#{reference}"
+    end
   end
 
   def reference_link_text(from_project = nil)
     self.title
   end
 
+  def title=(value)
+    write_attribute(:title, sanitize_title(value)) if value.present?
+  end
+
   def milestoneish_ids
     id
   end
@@ -130,10 +182,43 @@ class Milestone < ActiveRecord::Base
     nil
   end
 
+  def safe_title
+    title.to_slug.normalize.to_s
+  end
+
+  def parent
+    group || project
+  end
+
+  def is_group_milestone?
+    group_id.present?
+  end
+
+  def is_project_milestone?
+    project_id.present?
+  end
+
   private
 
+  # Milestone titles must be unique across project milestones and group milestones
   def uniqueness_of_title
-    super(project.group, project)
+    if project
+      title_exists = project.milestones.find_by_title(title)
+      title_exists ||= project.group.milestones.find_by_title(title) if project.group
+    elsif group
+      title_exists = group.milestones.find_by_title(title)
+      title_exists ||= Milestone.where(project: group.projects).find_by_title(title)
+    end
+
+    errors.add(:title, "already being used for another group or project milestone.") if title_exists
+  end
+
+  # Milestone should be either a project milestone or a group milestone
+  def milestone_type_check
+    if group_id && project_id
+      field = project_id_changed? ? :project_id : :group_id
+      errors.add(field, "milestone should belong either to a project or a group.")
+    end
   end
 
   def milestone_format_reference(format = :iid)
@@ -146,6 +231,16 @@ class Milestone < ActiveRecord::Base
     end
   end
 
+  def sanitize_title(value)
+    CGI.unescape_html(Sanitize.clean(value.to_s))
+  end
+
+  def start_date_should_be_less_than_due_date
+    if due_date <= start_date
+      errors.add(:start_date, "Can't be greater than due date")
+    end
+  end
+
   def issues_finder_params
     { project_id: project_id }
   end
diff --git a/app/models/namespace.rb b/app/models/namespace.rb
index a25234b886b..672eab94c07 100644
--- a/app/models/namespace.rb
+++ b/app/models/namespace.rb
@@ -16,7 +16,6 @@ class Namespace < ActiveRecord::Base
   cache_markdown_field :description, pipeline: :description
 
   has_many :projects, dependent: :destroy
-  has_many :groups, dependent: :destroy
   has_many :project_statistics
   belongs_to :owner, class_name: "User"
 
diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index 8dd0846f3bc..dbbc9ae273a 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -89,8 +89,10 @@ class IssuableBaseService < BaseService
     milestone_id = params[:milestone_id]
     return unless milestone_id
 
-    if milestone_id == IssuableFinder::NONE ||
-        project.milestones.find_by(id: milestone_id).nil?
+    params[:milestone_id] = '' if milestone_id == IssuableFinder::NONE
+
+    if project.milestones.find_by(id: milestone_id).nil? &&
+        project.group&.milestones&.find(milestone_id).nil?
       params[:milestone_id] = ''
     end
   end
diff --git a/app/services/issues/move_service.rb b/app/services/issues/move_service.rb
index 711f4035c55..020365fa19a 100644
--- a/app/services/issues/move_service.rb
+++ b/app/services/issues/move_service.rb
@@ -61,8 +61,16 @@ module Issues
     end
 
     def cloneable_milestone_id
-      @new_project.milestones
-        .find_by(title: @old_issue.milestone.try(:title)).try(:id)
+      title = @old_issue.milestone&.title
+      return unless title
+
+      milestone_id = @new_project.milestones.find_by(title: title).try(:id)
+
+      if milestone_id.nil? && @new_project.group && can?(current_user, :read_group, @new_project.group)
+        milestone_id = @new_project.group.milestones.find_by(title: title).try(:id)
+      end
+
+      milestone_id
     end
 
     def rewrite_notes
diff --git a/app/services/milestones/close_service.rb b/app/services/milestones/close_service.rb
index 608fc49d766..776ec4b287b 100644
--- a/app/services/milestones/close_service.rb
+++ b/app/services/milestones/close_service.rb
@@ -1,7 +1,7 @@
 module Milestones
   class CloseService < Milestones::BaseService
     def execute(milestone)
-      if milestone.close
+      if milestone.close && milestone.is_project_milestone?
         event_service.close_milestone(milestone, current_user)
       end
 
diff --git a/app/services/milestones/reopen_service.rb b/app/services/milestones/reopen_service.rb
index 573f9ee5c21..5b8b682caaf 100644
--- a/app/services/milestones/reopen_service.rb
+++ b/app/services/milestones/reopen_service.rb
@@ -1,7 +1,7 @@
 module Milestones
   class ReopenService < Milestones::BaseService
     def execute(milestone)
-      if milestone.activate
+      if milestone.activate && milestone.is_project_milestone?
         event_service.reopen_milestone(milestone, current_user)
       end
 
diff --git a/app/views/groups/milestones/_form.html.haml b/app/views/groups/milestones/_form.html.haml
new file mode 100644
index 00000000000..2715b708a9f
--- /dev/null
+++ b/app/views/groups/milestones/_form.html.haml
@@ -0,0 +1,29 @@
+= form_for [@group, @milestone], html: { class: 'form-horizontal milestone-form common-note-form js-quick-submit js-requires-input' }  do |f|
+  .row
+    = form_errors(@milestone)
+
+    = hidden_field_tag :title, @milestone&.title
+
+    .col-md-6
+      .form-group
+        = f.label :title, "Title", class: "control-label"
+        .col-sm-10
+          = f.text_field :title, maxlength: 255, class: "form-control", required: true, autofocus: true
+      .form-group.milestone-description
+        = f.label :description, "Description", class: "control-label"
+        .col-sm-10
+          = render layout: 'projects/md_preview', locals: { url: '' } do
+            = render 'projects/zen', f: f, attr: :description, classes: 'note-textarea', placeholder: 'Write milestone description...'
+            .clearfix
+            .error-alert
+
+    = render "shared/milestones/form_dates", f: f
+
+  .form-actions
+    - if @milestone.new_record?
+      = f.submit 'Create milestone', class: "btn-create btn"
+      = link_to "Cancel", group_milestones_path(@group), class: "btn btn-cancel"
+    - else
+      = f.submit 'Update milestone', class: "btn-create btn"
+      = link_to "Cancel", group_milestone_path(@group, @milestone), class: "btn btn-cancel"
+
diff --git a/app/views/groups/milestones/edit.html.haml b/app/views/groups/milestones/edit.html.haml
new file mode 100644
index 00000000000..5f6d7d209d0
--- /dev/null
+++ b/app/views/groups/milestones/edit.html.haml
@@ -0,0 +1,7 @@
+- page_title "Milestones"
+- render "header_title"
+
+%h3.page-title
+  Edit Milestone
+
+= render "form"
diff --git a/app/views/groups/milestones/index.html.haml b/app/views/groups/milestones/index.html.haml
index f91bee0b610..6ceb4092307 100644
--- a/app/views/groups/milestones/index.html.haml
+++ b/app/views/groups/milestones/index.html.haml
@@ -9,11 +9,6 @@
       = link_to new_group_milestone_path(@group), class: "btn btn-new" do
         New milestone
 
-.row-content-block
-  Only milestones from
-  %strong= @group.name
-  group are listed here.
-
 .milestones
   %ul.content-list
     - if @milestones.blank?
diff --git a/app/views/groups/milestones/new.html.haml b/app/views/groups/milestones/new.html.haml
index 7c7573862d0..e24844661ee 100644
--- a/app/views/groups/milestones/new.html.haml
+++ b/app/views/groups/milestones/new.html.haml
@@ -4,40 +4,4 @@
 %h3.page-title
   New Milestone
 
-%p.light
-  This will create milestone in every selected project
-%hr
-
-= form_for @milestone, url: group_milestones_path(@group), html: { class: 'form-horizontal milestone-form common-note-form js-quick-submit js-requires-input' }  do |f|
-  .row
-    - if @milestone.errors.any?
-      #error_explanation
-        .alert.alert-danger
-          %ul
-            - @milestone.errors.full_messages.each do |msg|
-              %li
-                = msg
-
-    .col-md-6
-      .form-group
-        = f.label :title, "Title", class: "control-label"
-        .col-sm-10
-          = f.text_field :title, maxlength: 255, class: "form-control", required: true, autofocus: true
-      .form-group.milestone-description
-        = f.label :description, "Description", class: "control-label"
-        .col-sm-10
-          = render layout: 'projects/md_preview', locals: { url: '' } do
-            = render 'projects/zen', f: f, attr: :description, classes: 'note-textarea', placeholder: 'Write milestone description...'
-            .clearfix
-            .error-alert
-      .form-group
-        = f.label :projects, "Projects", class: "control-label"
-        .col-sm-10
-          = f.collection_select :project_ids, @group.projects.non_archived, :id, :name,
-            { selected: @group.projects.non_archived.pluck(:id) }, required: true, multiple: true, class: 'select2'
-
-    = render "shared/milestones/form_dates", f: f
-
-  .form-actions
-    = f.submit 'Create milestone', class: "btn-create btn"
-    = link_to "Cancel", group_milestones_path(@group), class: "btn btn-cancel"
+= render "form"
diff --git a/app/views/groups/milestones/show.html.haml b/app/views/groups/milestones/show.html.haml
index 33e68bc766e..54b1b7a734a 100644
--- a/app/views/groups/milestones/show.html.haml
+++ b/app/views/groups/milestones/show.html.haml
@@ -1,4 +1,4 @@
 = render "header_title"
 = render 'shared/milestones/top', milestone: @milestone, group: @group
-= render 'shared/milestones/tabs', milestone: @milestone, show_project_name: true
+= render 'shared/milestones/tabs', milestone: @milestone, show_project_name: true if @milestone.is_legacy_group_milestone?
 = render 'shared/milestones/sidebar', milestone: @milestone, affix_offset: 102
diff --git a/app/views/shared/milestones/_milestone.html.haml b/app/views/shared/milestones/_milestone.html.haml
index d0acf575cbe..2294d37aa4d 100644
--- a/app/views/shared/milestones/_milestone.html.haml
+++ b/app/views/shared/milestones/_milestone.html.haml
@@ -1,10 +1,15 @@
 - dashboard = local_assigns[:dashboard]
-- custom_dom_id = "milestone_#{milestone.title}"
+- custom_dom_id = dom_id(milestone.try(:milestones) ? milestone.milestones.first : milestone)
 
 %li{ class: "milestone milestone-#{milestone.closed? ? 'closed' : 'open'}", id: custom_dom_id }
   .row
     .col-sm-6
       %strong= link_to truncate(milestone.title, length: 100), milestone_path
+      - if milestone.is_group_milestone?
+        %span - Group Milestone
+      - else
+        %span - Project Milestone
+
     .col-sm-6
       .pull-right.light #{milestone.percent_complete(current_user)}% complete
   .row
@@ -13,26 +18,32 @@
       &middot;
       = link_to pluralize(milestone.merge_requests.size, 'Merge Request'), merge_requests_path
     .col-sm-6= milestone_progress_bar(milestone)
-  - if milestone.is_a?(GlobalMilestone)
+  - if milestone.is_a?(GlobalMilestone) || milestone.is_group_milestone?
     .row
       .col-sm-6
-        .expiration= render('shared/milestone_expired', milestone: milestone)
-        .projects
-          - milestone.milestones.each do |milestone|
-            = link_to milestone_path(milestone) do
-              %span.label.label-gray
-                = dashboard ? milestone.project.name_with_namespace : milestone.project.name
+        - if milestone.is_legacy_group_milestone?
+          .expiration= render('shared/milestone_expired', milestone: milestone)
+          .projects
+            - milestone.milestones.each do |milestone|
+              = link_to milestone_path(milestone) do
+                %span.label.label-gray
+                  = dashboard ? milestone.project.name_with_namespace : milestone.project.name
       - if @group
-        .col-sm-6
+        .col-sm-6.milestone-actions
           - if can?(current_user, :admin_milestones, @group)
+            - if milestone.is_group_milestone?
+              = link_to edit_group_milestone_path(@group, milestone.safe_title, title: milestone.title), class: "btn btn-xs btn-grouped" do
+                Edit
+              \
             - if milestone.closed?
               = link_to 'Reopen Milestone', group_milestone_path(@group, milestone.safe_title, title: milestone.title, milestone: {state_event: :activate }), method: :put, class: "btn btn-xs btn-grouped btn-reopen"
             - else
-              = link_to 'Close Milestone', group_milestone_path(@group, milestone.safe_title, title: milestone.title, milestone: {state_event: :close }), method: :put, class: "btn btn-xs btn-close"
+              = link_to 'Close Milestone', group_milestone_path(@group, milestone.safe_title, title: milestone.title, milestone: {state_event: :close }), method: :put, class: "btn btn-xs btn-grouped btn-close"
 
   - if @project
     .row
-      .col-sm-6= render('shared/milestone_expired', milestone: milestone)
+      .col-sm-6
+        = render('shared/milestone_expired', milestone: milestone)
       .col-sm-6.milestone-actions
         - if can?(current_user, :admin_milestone, milestone.project) and milestone.active?
           = link_to edit_namespace_project_milestone_path(milestone.project.namespace, milestone.project, milestone), class: "btn btn-xs btn-grouped" do
diff --git a/app/views/shared/milestones/_top.html.haml b/app/views/shared/milestones/_top.html.haml
index 58ea0a356ae..149dfdb9158 100644
--- a/app/views/shared/milestones/_top.html.haml
+++ b/app/views/shared/milestones/_top.html.haml
@@ -22,6 +22,9 @@
   - if group
     .pull-right
       - if can?(current_user, :admin_milestones, group)
+        - if milestone.is_group_milestone?
+          = link_to edit_group_milestone_path(group, milestone.safe_title, title: milestone.title), class: "btn btn btn-grouped" do
+            Edit
         - if milestone.active?
           = link_to 'Close Milestone', group_milestone_path(group, milestone.safe_title, title: milestone.title, milestone: {state_event: :close }), method: :put, class: "btn btn-grouped btn-close"
         - else
@@ -30,32 +33,44 @@
 .detail-page-description.milestone-detail
   %h2.title
     = markdown_field(milestone, :title)
+  - if @milestone.is_group_milestone? && @milestone.description.present?
+    %div
+      .description
+        .wiki
+          = markdown_field(@milestone, :description)
 
 - if milestone.complete?(current_user) && milestone.active?
   .alert.alert-success.prepend-top-default
     - close_msg = group ? 'You may close the milestone now.' : 'Navigate to the project to close the milestone.'
     %span All issues for this milestone are closed. #{close_msg}
 
-%h2 Only link to group milestone issues instead of list(for now)
-/ .table-holder
-/   %table.table
-/     %thead
-/       %tr
-/         %th Project
-/         %th Open issues
-/         %th State
-/         %th Due date
-/     - milestone.milestones.each do |ms|
-/       %tr
-/         %td
-/           - project_name = group ? ms.project.name : ms.project.name_with_namespace
-/           = link_to project_name, namespace_project_milestone_path(ms.project.namespace, ms.project, ms)
-/         %td
-/           = ms.issues_visible_to_user(current_user).opened.count
-/         %td
-/           - if ms.closed?
-/             Closed
-/           - else
-/             Open
-/         %td
-/           = ms.expires_at
+- if @milestone.is_legacy_group_milestone?
+  .table-holder
+    %table.table
+      %thead
+        %tr
+          %th Project
+          %th Open issues
+          %th State
+          %th Due date
+      - milestone.milestones.each do |ms|
+        %tr
+          %td
+            - project_name = group ? ms.project.name : ms.project.name_with_namespace
+            = link_to project_name, namespace_project_milestone_path(ms.project.namespace, ms.project, ms)
+          %td
+            = ms.issues_visible_to_user(current_user).opened.count
+          %td
+            - if ms.closed?
+              Closed
+            - else
+              Open
+          %td
+            = ms.expires_at
+- elsif @milestone.is_group_milestone?
+  %br
+  View
+  = link_to 'Issues', issues_group_path(@group, milestone_title: milestone.title)
+  or
+  = link_to 'Merge Requests', merge_requests_group_path(@group, milestone_title: milestone.title)
+  in this milestone
diff --git a/changelogs/unreleased/issue_30126_be.yml b/changelogs/unreleased/issue_30126_be.yml
new file mode 100644
index 00000000000..96bb8d9574b
--- /dev/null
+++ b/changelogs/unreleased/issue_30126_be.yml
@@ -0,0 +1,4 @@
+---
+title: Add native group milestones
+merge_request:
+author:
diff --git a/config/routes/group.rb b/config/routes/group.rb
index 11cdff55ed8..47010401191 100644
--- a/config/routes/group.rb
+++ b/config/routes/group.rb
@@ -12,7 +12,7 @@ scope(path: 'groups/*group_id',
   end
 
   resource :avatar, only: [:destroy]
-  resources :milestones, constraints: { id: /[^\/]+/ }, only: [:index, :show, :update, :new, :create] do
+  resources :milestones, constraints: { id: /[^\/]+/ }, only: [:index, :show, :edit, :update, :new, :create] do
     member do
       get :merge_requests
       get :participants
diff --git a/db/migrate/20170619183807_create_group_milestones.rb b/db/migrate/20170619183807_create_group_milestones.rb
deleted file mode 100644
index 9ce287bb306..00000000000
--- a/db/migrate/20170619183807_create_group_milestones.rb
+++ /dev/null
@@ -1,23 +0,0 @@
-class CreateGroupMilestones < ActiveRecord::Migration
-  DOWNTIME = false
-
-  def change
-    create_table :group_milestones do |t|
-      t.integer :group_id
-      t.string :title
-      t.text :description
-      t.date :start_date
-      t.date :due_date
-      t.string :state
-      t.string :title_html
-      t.string :description_html
-      t.integer :cached_markdown_version, limit: 4
-    end
-
-    add_column :issues, :group_milestone_id, :integer
-    add_column :merge_requests, :group_milestone_id, :integer
-
-    add_foreign_key :group_milestones, :namespaces, column: :group_id, null: false # rubocop: disable Migration/AddConcurrentForeignKey
-    add_index :group_milestones, :group_id
-  end
-end
diff --git a/db/migrate/20170619184243_add_group_milestone_group_id_indexes.rb b/db/migrate/20170619184243_add_group_milestone_group_id_indexes.rb
new file mode 100644
index 00000000000..2bdea582b86
--- /dev/null
+++ b/db/migrate/20170619184243_add_group_milestone_group_id_indexes.rb
@@ -0,0 +1,19 @@
+class AddGroupMilestoneGroupIdIndexes < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  DOWNTIME = false
+
+  def up
+    add_concurrent_foreign_key :milestones, :namespaces, column: :group_id, on_delete: :cascade
+
+    add_concurrent_index :milestones, :group_id
+  end
+
+  def down
+    remove_foreign_key :milestones, column: :group_id
+
+    remove_concurrent_index :milestones, :group_id
+  end
+end
diff --git a/db/migrate/20170623183807_add_group_id_to_milestones.rb b/db/migrate/20170623183807_add_group_id_to_milestones.rb
new file mode 100644
index 00000000000..ab98905ac53
--- /dev/null
+++ b/db/migrate/20170623183807_add_group_id_to_milestones.rb
@@ -0,0 +1,18 @@
+class AddGroupIdToMilestones < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    change_column_null :milestones, :project_id, true
+
+    add_column :milestones, :group_id, :integer
+  end
+
+  def down
+    remove_column :milestones, :group_id
+    change_column_null :milestones, :project_id, true
+
+    # We cannot rollback project_id not null constraint if there are records
+    # with null values.
+    execute "DELETE from milestones WHERE project_id IS NULL"
+  end
+end
diff --git a/db/migrate/20170624184243_add_group_milestone_id_indexes.rb b/db/migrate/20170624184243_add_group_milestone_id_indexes.rb
index 836d39e6e6d..6289d6d894f 100644
--- a/db/migrate/20170624184243_add_group_milestone_id_indexes.rb
+++ b/db/migrate/20170624184243_add_group_milestone_id_indexes.rb
@@ -6,18 +6,14 @@ class AddGroupMilestoneIdIndexes < ActiveRecord::Migration
   DOWNTIME = false
 
   def up
-    add_foreign_key :issues, :namespaces, column: :group_milestone_id
-    add_foreign_key :merge_requests, :namespaces, column: :group_milestone_id
+    add_concurrent_foreign_key :milestones, :namespaces, column: :group_id
 
-    add_concurrent_index :issues, :group_milestone_id
-    add_concurrent_index :merge_requests, :group_milestone_id
+    add_concurrent_index :milestones, :group_id
   end
 
   def down
-    remove_foreign_key :issues, column: :group_milestone_id
-    remove_foreign_key :merge_requests, column: :group_milestone_id
+    remove_foreign_key :milestones, column: :group_id
 
-    remove_concurrent_index :issues, :group_milestone_id
-    remove_concurrent_index :merge_requests, :group_milestone_id
+    remove_concurrent_index :milestones, :group_id
   end
 end
diff --git a/db/schema.rb b/db/schema.rb
index eec2e91c3c6..6d2fe1e47ea 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -771,14 +771,7 @@ ActiveRecord::Schema.define(version: 20170624184243) do
     t.datetime "last_edited_at"
     t.integer "last_edited_by_id"
     t.integer "head_pipeline_id"
-<<<<<<< bce6d12bb9365ec2e8ffd5f40f0d1f03f88b315a
-<<<<<<< 5af1fcd6f329858d757bab0d67cb50af6c820160
     t.boolean "ref_fetched"
-=======
-    t.integer "group_milestone_id"
->>>>>>> Add group milestones table
-=======
->>>>>>> Native group milestones
   end
 
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree
@@ -786,14 +779,7 @@ ActiveRecord::Schema.define(version: 20170624184243) do
   add_index "merge_requests", ["created_at"], name: "index_merge_requests_on_created_at", using: :btree
   add_index "merge_requests", ["deleted_at"], name: "index_merge_requests_on_deleted_at", using: :btree
   add_index "merge_requests", ["description"], name: "index_merge_requests_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
-<<<<<<< bce6d12bb9365ec2e8ffd5f40f0d1f03f88b315a
-<<<<<<< 5af1fcd6f329858d757bab0d67cb50af6c820160
   add_index "merge_requests", ["head_pipeline_id"], name: "index_merge_requests_on_head_pipeline_id", using: :btree
-=======
-  add_index "merge_requests", ["group_milestone_id"], name: "index_merge_requests_on_group_milestone_id", using: :btree
->>>>>>> Add group milestones table
-=======
->>>>>>> Native group milestones
   add_index "merge_requests", ["milestone_id"], name: "index_merge_requests_on_milestone_id", using: :btree
   add_index "merge_requests", ["source_branch"], name: "index_merge_requests_on_source_branch", using: :btree
   add_index "merge_requests", ["source_project_id"], name: "index_merge_requests_on_source_project_id", using: :btree
diff --git a/features/group/milestones.feature b/features/group/milestones.feature
index 1c1539b3e12..2211acfee20 100644
--- a/features/group/milestones.feature
+++ b/features/group/milestones.feature
@@ -22,12 +22,12 @@ Feature: Group Milestones
     Then I should see group milestone with descriptions and expiry date
     And I should see group milestone with all issues and MRs assigned to that milestone
 
-  Scenario: Create multiple milestones with one form
+  Scenario: Create group milestones
     Given I visit group "Owned" milestones page
     And I click new milestone button
     And I fill milestone name
     When I press create mileston button
-    Then milestone in each project should be created
+    Then group milestone should be created
 
   Scenario: I should see Issues listed with labels
     Given Group has projects with milestones
diff --git a/features/steps/group/milestones.rb b/features/steps/group/milestones.rb
index 0542b06c0ab..03910c00729 100644
--- a/features/steps/group/milestones.rb
+++ b/features/steps/group/milestones.rb
@@ -54,14 +54,9 @@ class Spinach::Features::GroupMilestones < Spinach::FeatureSteps
     click_button "Create milestone"
   end
 
-  step 'milestone in each project should be created' do
+  step 'group milestone should be created' do
     group = Group.find_by(name: 'Owned')
-    expect(page).to have_content "Milestone v2.9.0"
-    expect(group.projects).to be_present
-
-    group.projects.each do |project|
-      expect(page).to have_content project.name
-    end
+    expect(page).to have_content group.milestones.find_by_title('v2.9.0').title
   end
 
   step 'I should see the "bug" label' do
diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index cef5a0abe12..d1068fff03b 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -266,7 +266,12 @@ module API
       expose :deleted_file?, as: :deleted_file
     end
 
-    class Milestone < ProjectEntity
+    class Milestone < Grape::Entity
+      expose :id, :iid
+      expose(:project_id) { |entity| entity&.project&.id }
+      expose(:group_id) { |entity| entity&.group&.id }
+      expose :title, :description
+      expose :state, :created_at, :updated_at
       expose :due_date
       expose :start_date
     end
diff --git a/spec/controllers/groups/milestones_controller_spec.rb b/spec/controllers/groups/milestones_controller_spec.rb
index c6e5fb61cf9..67c70af9ad1 100644
--- a/spec/controllers/groups/milestones_controller_spec.rb
+++ b/spec/controllers/groups/milestones_controller_spec.rb
@@ -17,6 +17,14 @@ describe Groups::MilestonesController do
   end
   let(:milestone_path) { group_milestone_path(group, milestone.safe_title, title: milestone.title) }
 
+  let(:milestone_params) do
+    {
+      title: title,
+      start_date: Date.today,
+      due_date: 1.month.from_now.to_date
+    }
+  end
+
   before do
     sign_in(user)
     group.add_owner(user)
@@ -44,16 +52,31 @@ describe Groups::MilestonesController do
     it "creates group milestone with Chinese title" do
       post :create,
            group_id: group.to_param,
-           milestone: { project_ids: [project.id, project2.id], title: title }
+           milestone: milestone_params
 
+      milestone = Milestone.find_by_title(title)
       expect(response).to redirect_to(group_milestone_path(group, title.to_slug.to_s, title: title))
-      expect(Milestone.where(title: title).count).to eq(2)
+      expect(milestone.group_id).to eq(group.id)
+      expect(milestone.due_date).to eq(milestone_params[:due_date])
+      expect(milestone.start_date).to eq(milestone_params[:start_date])
     end
+  end
+
+  describe "#update" do
+    let(:milestone) { create(:milestone, group: group) }
+
+    it "updates group milestone" do
+      milestone_params[:title] = "title changed"
+
+      put :update,
+           id: milestone.title.to_slug.to_s,
+           group_id: group.to_param,
+           milestone: milestone_params,
+           title: milestone.title
 
-    it "redirects to new when there are no project ids" do
-      post :create, group_id: group.to_param, milestone: { title: title, project_ids: [""] }
-      expect(response).to render_template :new
-      expect(assigns(:milestone).errors).not_to be_nil
+      milestone.reload
+      expect(response).to redirect_to(group_milestone_path(group, milestone.title.to_slug.to_s, title: milestone.title))
+      expect(milestone.title).to eq("title changed")
     end
   end
 
@@ -156,7 +179,7 @@ describe Groups::MilestonesController do
       it 'does not 404' do
         post :create,
              group_id: group.to_param,
-             milestone: { project_ids: [project.id, project2.id], title: title }
+             milestone: { title: title }
 
         expect(response).not_to have_http_status(404)
       end
@@ -164,7 +187,7 @@ describe Groups::MilestonesController do
       it 'does not redirect to the correct casing' do
         post :create,
              group_id: group.to_param,
-             milestone: { project_ids: [project.id, project2.id], title: title }
+             milestone: { title: title }
 
         expect(response).not_to have_http_status(301)
       end
@@ -176,7 +199,7 @@ describe Groups::MilestonesController do
       it 'returns not found' do
         post :create,
              group_id: redirect_route.path,
-             milestone: { project_ids: [project.id, project2.id], title: title }
+             milestone: { title: title }
 
         expect(response).to have_http_status(404)
       end
diff --git a/spec/controllers/projects/milestones_controller_spec.rb b/spec/controllers/projects/milestones_controller_spec.rb
index 84a61b2784e..5af36df7405 100644
--- a/spec/controllers/projects/milestones_controller_spec.rb
+++ b/spec/controllers/projects/milestones_controller_spec.rb
@@ -31,6 +31,39 @@ describe Projects::MilestonesController do
     end
   end
 
+  describe "#index" do
+    context "as html" do
+      before do
+        get :index, namespace_id: project.namespace.id, project_id: project.id
+      end
+
+      it "queries only projects milestones" do
+        milestones = assigns(:milestones)
+        expect(milestones.count).to eq(1)
+        expect(milestones.where(project_id: nil)).to be_empty
+      end
+    end
+
+    context "as json" do
+      let!(:group) { create(:group, :public) }
+      let!(:group_milestone) { create(:milestone, group: group) }
+      let!(:group_member) { create(:group_member, group: group, user: user) }
+
+      before do
+        project.update(namespace: group)
+        get :index, namespace_id: project.namespace.id, project_id: project.id, format: :json
+      end
+
+      it "queries projects milestones and groups milestones" do
+        milestones = assigns(:milestones)
+
+        expect(milestones.count).to eq(2)
+        expect(milestones.where(project_id: nil).first).to eq(group_milestone)
+        expect(milestones.where(group_id: nil).first).to eq(milestone)
+      end
+    end
+  end
+
   describe "#destroy" do
     it "removes milestone" do
       expect(issue.milestone_id).to eq(milestone.id)
diff --git a/spec/factories/milestones.rb b/spec/factories/milestones.rb
index 841ab3c73b8..113665ff11b 100644
--- a/spec/factories/milestones.rb
+++ b/spec/factories/milestones.rb
@@ -1,7 +1,13 @@
 FactoryGirl.define do
   factory :milestone do
     title
-    project factory: :empty_project
+
+    transient do
+      project nil
+      group nil
+      project_id nil
+      group_id nil
+    end
 
     trait :active do
       state "active"
@@ -11,6 +17,20 @@ FactoryGirl.define do
       state "closed"
     end
 
+    after(:build) do |milestone, evaluator|
+      if evaluator.group
+        milestone.group = evaluator.group
+      elsif evaluator.group_id
+        milestone.group_id = evaluator.group_id
+      elsif evaluator.project
+        milestone.project = evaluator.project
+      elsif evaluator.project_id
+        milestone.project_id = evaluator.project_id
+      else
+        milestone.project = create(:empty_project)
+      end
+    end
+
     factory :active_milestone, traits: [:active]
     factory :closed_milestone, traits: [:closed]
   end
diff --git a/spec/features/groups/milestone_spec.rb b/spec/features/groups/milestone_spec.rb
index 330310eae6b..9b6eb946f4b 100644
--- a/spec/features/groups/milestone_spec.rb
+++ b/spec/features/groups/milestone_spec.rb
@@ -33,4 +33,32 @@ feature 'Group milestones', :feature, :js do
       expect(find('.start_date')).to have_content(Date.today.at_beginning_of_month.strftime('%b %-d, %Y'))
     end
   end
+
+  context 'milestones list' do
+    let!(:other_project) { create(:project_empty_repo, group: group) }
+
+    let!(:active_group_milestone) { create(:milestone, group: group, state: 'active') }
+    let!(:active_project_milestone1) { create(:milestone, project: project, state: 'active', title: 'v1.0') }
+    let!(:active_project_milestone2) { create(:milestone, project: other_project, state: 'active', title: 'v1.0') }
+    let!(:closed_group_milestone) { create(:milestone, group: group, state: 'closed') }
+    let!(:closed_project_milestone1) { create(:milestone, project: project, state: 'closed', title: 'v2.0') }
+    let!(:closed_project_milestone2) { create(:milestone, project: other_project, state: 'closed', title: 'v2.0') }
+
+    before do
+      visit group_milestones_path(group)
+    end
+
+    it 'counts milestones correctly' do
+      expect(find('.top-area .active .badge').text).to eq("2")
+      expect(find('.top-area .closed .badge').text).to eq("2")
+      expect(find('.top-area .all .badge').text).to eq("4")
+    end
+
+    it 'lists legacy group milestones and group milestones' do
+      legacy_milestone = GroupMilestone.build_collection(group, group.projects, { state: 'active' }).first
+
+      expect(page).to have_selector("#milestone_#{active_group_milestone.id}", count: 1)
+      expect(page).to have_selector("#milestone_#{legacy_milestone.milestones.first.id}", count: 1)
+    end
+  end
 end
diff --git a/spec/features/milestone_spec.rb b/spec/features/milestone_spec.rb
index 58989581ffe..4a3ce55c71a 100644
--- a/spec/features/milestone_spec.rb
+++ b/spec/features/milestone_spec.rb
@@ -1,7 +1,8 @@
 require 'rails_helper'
 
 feature 'Milestone', feature: true do
-  let(:project) { create(:empty_project, :public) }
+  let(:group) { create(:group, :public) }
+  let(:project) { create(:empty_project, :public, namespace: group) }
   let(:user)   { create(:user) }
 
   before do
@@ -37,8 +38,8 @@ feature 'Milestone', feature: true do
     end
   end
 
-  feature 'Open a milestone with an existing title' do
-    scenario 'displays validation message' do
+  feature 'Open a project milestone with an existing title' do
+    scenario 'displays validation message when there is a project milestone with same title' do
       milestone = create(:milestone, project: project, title: 8.7)
 
       visit new_namespace_project_milestone_path(project.namespace, project)
@@ -47,7 +48,19 @@ feature 'Milestone', feature: true do
       end
       find('input[name="commit"]').click
 
-      expect(find('.alert-danger')).to have_content('Title has already been taken')
+      expect(find('.alert-danger')).to have_content('already being used for another group or project milestone.')
+    end
+
+    scenario 'displays validation message when there is a group milestone with same title' do
+      milestone = create(:milestone, project_id: nil, group: project.group, title: 8.7)
+
+      visit new_namespace_project_milestone_path(project.namespace, project)
+      page.within '.milestone-form' do
+        fill_in "milestone_title", with: milestone.title
+      end
+      find('input[name="commit"]').click
+
+      expect(find('.alert-danger')).to have_content('already being used for another group or project milestone.')
     end
   end
 end
diff --git a/spec/finders/issues_finder_spec.rb b/spec/finders/issues_finder_spec.rb
index 4a52f0d5c58..bef4fd44331 100644
--- a/spec/finders/issues_finder_spec.rb
+++ b/spec/finders/issues_finder_spec.rb
@@ -59,6 +59,23 @@ describe IssuesFinder do
         end
       end
 
+      context 'filtering by group milestone' do
+        let!(:group) { create(:group, :public) }
+        let(:group_milestone) { create(:milestone, group: group) }
+        let!(:group_member) { create(:group_member, group: group, user: user) }
+        let(:params) { { milestone_title: group_milestone.title } }
+
+        before do
+          project2.update(namespace: group)
+          issue2.update(milestone: group_milestone)
+          issue3.update(milestone: group_milestone)
+        end
+
+        it 'returns issues assigned to that group milestone' do
+          expect(issues).to contain_exactly(issue2, issue3)
+        end
+      end
+
       context 'filtering by no milestone' do
         let(:params) { { milestone_title: Milestone::None.title } }
 
diff --git a/spec/finders/merge_requests_finder_spec.rb b/spec/finders/merge_requests_finder_spec.rb
index 5eb26de6c92..b46218bf72e 100644
--- a/spec/finders/merge_requests_finder_spec.rb
+++ b/spec/finders/merge_requests_finder_spec.rb
@@ -47,6 +47,25 @@ describe MergeRequestsFinder do
       expect(merge_requests).to contain_exactly(merge_request1)
     end
 
+    context 'filtering by group milestone' do
+      let!(:group) { create(:group, :public) }
+      let(:group_milestone) { create(:milestone, group: group) }
+      let!(:group_member) { create(:group_member, group: group, user: user) }
+      let(:params) { { milestone_title: group_milestone.title } }
+
+      before do
+        project2.update(namespace: group)
+        merge_request2.update(milestone: group_milestone)
+        merge_request3.update(milestone: group_milestone)
+      end
+
+      it 'returns issues assigned to that group milestone' do
+        merge_requests = described_class.new(user, params).execute
+
+        expect(merge_requests).to contain_exactly(merge_request2, merge_request3)
+      end
+    end
+
     context 'with created_after and created_before params' do
       let(:project4) { create(:empty_project, forked_from_project: project1) }
 
diff --git a/spec/finders/milestones_finder_spec.rb b/spec/finders/milestones_finder_spec.rb
new file mode 100644
index 00000000000..e81a5bae9a9
--- /dev/null
+++ b/spec/finders/milestones_finder_spec.rb
@@ -0,0 +1,48 @@
+require 'spec_helper'
+
+describe MilestonesFinder do
+  let(:group) { create(:group) }
+  let(:project_1) { create(:empty_project, namespace: group) }
+  let(:project_2) { create(:empty_project, namespace: group) }
+  let!(:milestone_1) { create(:milestone, group: group) }
+  let!(:milestone_2) { create(:milestone, group: group) }
+  let!(:milestone_3) { create(:milestone, project: project_1, state: 'active') }
+  let!(:milestone_4) { create(:milestone, project: project_2, state: 'active') }
+
+  it 'it returns milestones for projects' do
+    result = described_class.new(projects: [project_1, project_2], params: { state: 'all' }).execute
+
+    expect(result).to contain_exactly(milestone_3, milestone_4)
+  end
+
+  it 'returns milestones for groups' do
+    result = described_class.new(groups: group, params: { state: 'all' }).execute
+
+    expect(result).to contain_exactly(milestone_1, milestone_2)
+  end
+
+  it 'returns milestones for groups and projects' do
+    result = described_class.new(projects: [project_1, project_2], groups: group, params: { state: 'all' }).execute
+
+    expect(result).to contain_exactly(milestone_1, milestone_2, milestone_3, milestone_4)
+  end
+
+  context 'state filtering' do
+    before do
+      milestone_1.close
+      milestone_3.close
+    end
+
+    it 'filters by active state' do
+      result = described_class.new(projects: [project_1, project_2], groups: group, params: { state: 'active' }).execute
+
+      expect(result).to contain_exactly(milestone_2, milestone_4)
+    end
+
+    it 'filters by closed state' do
+      result = described_class.new(projects: [project_1, project_2], groups: group, params: { state: 'closed' }).execute
+
+      expect(result).to contain_exactly(milestone_1, milestone_3)
+    end
+  end
+end
diff --git a/spec/fixtures/api/schemas/public_api/v3/issues.json b/spec/fixtures/api/schemas/public_api/v3/issues.json
index f2ee9c925ae..51b0822bc66 100644
--- a/spec/fixtures/api/schemas/public_api/v3/issues.json
+++ b/spec/fixtures/api/schemas/public_api/v3/issues.json
@@ -22,7 +22,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/fixtures/api/schemas/public_api/v3/merge_requests.json b/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
index 01f9fbb2c89..b5c74bcc26e 100644
--- a/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
+++ b/spec/fixtures/api/schemas/public_api/v3/merge_requests.json
@@ -53,7 +53,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/fixtures/api/schemas/public_api/v4/issues.json b/spec/fixtures/api/schemas/public_api/v4/issues.json
index 2d1c84ee93d..bd6bfc03199 100644
--- a/spec/fixtures/api/schemas/public_api/v4/issues.json
+++ b/spec/fixtures/api/schemas/public_api/v4/issues.json
@@ -22,7 +22,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/fixtures/api/schemas/public_api/v4/merge_requests.json b/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
index 51642e8cbb8..60aa47c1259 100644
--- a/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
+++ b/spec/fixtures/api/schemas/public_api/v4/merge_requests.json
@@ -53,7 +53,8 @@
         "properties": {
           "id": { "type": "integer" },
           "iid": { "type": "integer" },
-          "project_id": { "type": "integer" },
+          "project_id": { "type": ["integer", "null"] },
+          "group_id": { "type": ["integer", "null"] },
           "title": { "type": "string" },
           "description": { "type": ["string", "null"] },
           "state": { "type": "string" },
diff --git a/spec/lib/gitlab/import_export/all_models.yml b/spec/lib/gitlab/import_export/all_models.yml
index a5f09f1856e..5df4fc2de84 100644
--- a/spec/lib/gitlab/import_export/all_models.yml
+++ b/spec/lib/gitlab/import_export/all_models.yml
@@ -45,6 +45,7 @@ label:
 - merge_requests
 - priorities
 milestone:
+- group
 - project
 - issues
 - labels
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index fadd3ad1330..1c40984e863 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -82,6 +82,7 @@ Milestone:
 - id
 - title
 - project_id
+- group_id
 - description
 - due_date
 - start_date
diff --git a/spec/models/milestone_spec.rb b/spec/models/milestone_spec.rb
index 45953023a36..6c06c10e7ce 100644
--- a/spec/models/milestone_spec.rb
+++ b/spec/models/milestone_spec.rb
@@ -6,9 +6,6 @@ describe Milestone, models: true do
       allow(subject).to receive(:set_iid).and_return(false)
     end
 
-    it { is_expected.to validate_presence_of(:title) }
-    it { is_expected.to validate_presence_of(:project) }
-
     describe 'start_date' do
       it 'adds an error when start_date is greated then due_date' do
         milestone = build(:milestone, start_date: Date.tomorrow, due_date: Date.yesterday)
@@ -37,17 +34,42 @@ describe Milestone, models: true do
     end
   end
 
-  describe "unique milestone title per project" do
-    it "does not accept the same title in a project twice" do
-      new_milestone = Milestone.new(project: milestone.project, title: milestone.title)
-      expect(new_milestone).not_to be_valid
+  describe "unique milestone title" do
+    context "per project" do
+      it "does not accept the same title in a project twice" do
+        new_milestone = Milestone.new(project: milestone.project, title: milestone.title)
+        expect(new_milestone).not_to be_valid
+      end
+
+      it "accepts the same title in another project" do
+        project = build(:empty_project)
+        new_milestone = Milestone.new(project: project, title: milestone.title)
+
+        expect(new_milestone).to be_valid
+      end
     end
 
-    it "accepts the same title in another project" do
-      project = build(:empty_project)
-      new_milestone = Milestone.new(project: project, title: milestone.title)
+    context "per group" do
+      let(:group) { create(:group) }
+      let(:milestone) { create(:milestone, group: group) }
+
+      before do
+        project.update(group: group)
+      end
+
+      it "does not accept the same title in a group twice" do
+        new_milestone = Milestone.new(group: group, title: milestone.title)
+
+        expect(new_milestone).not_to be_valid
+      end
 
-      expect(new_milestone).to be_valid
+      it "does not accept the same title of a child project milestone" do
+        create(:milestone, project: group.projects.first)
+
+        new_milestone = Milestone.new(group: group, title: milestone.title)
+
+        expect(new_milestone).not_to be_valid
+      end
     end
   end
 
diff --git a/spec/services/issues/move_service_spec.rb b/spec/services/issues/move_service_spec.rb
index d1dd1466d95..36d5038fb95 100644
--- a/spec/services/issues/move_service_spec.rb
+++ b/spec/services/issues/move_service_spec.rb
@@ -37,9 +37,6 @@ describe Issues::MoveService, services: true do
 
   describe '#execute' do
     shared_context 'issue move executed' do
-      let!(:milestone2) do
-        create(:milestone, project_id: new_project.id, title: 'v9.0')
-      end
       let!(:award_emoji) { create(:award_emoji, awardable: old_issue) }
 
       let!(:new_issue) { move_service.execute(old_issue, new_project) }
@@ -48,6 +45,63 @@ describe Issues::MoveService, services: true do
     context 'issue movable' do
       include_context 'user can move issue'
 
+      context 'move to new milestone'  do
+        let(:new_issue) { move_service.execute(old_issue, new_project) }
+
+        context 'project milestone' do
+          let!(:milestone2) do
+            create(:milestone, project_id: new_project.id, title: 'v9.0')
+          end
+
+          it 'assigns milestone to new issue' do
+            expect(new_issue.reload.milestone.title).to eq 'v9.0'
+            expect(new_issue.reload.milestone).to eq(milestone2)
+          end
+        end
+
+        context 'group milestones' do
+          let!(:group) { create(:group, :private) }
+          let!(:group_milestone_1) do
+            create(:milestone, group_id: group.id, title: 'v9.0_group')
+          end
+
+          before do
+            old_issue.update(milestone: group_milestone_1)
+            old_project.update(namespace: group)
+            new_project.update(namespace: group)
+
+            group.add_users([user], GroupMember::DEVELOPER)
+          end
+
+          context 'when moving to a project of the same group' do
+            it 'keeps the same group milestone' do
+              expect(new_issue.reload.project).to eq(new_project)
+              expect(new_issue.reload.milestone).to eq(group_milestone_1)
+            end
+          end
+
+          context 'when moving to a project of a different group' do
+            let!(:group_2) { create(:group, :private) }
+
+            let!(:group_milestone_2) do
+              create(:milestone, group_id: group_2.id, title: 'v9.0_group')
+            end
+
+            before do
+              old_issue.update(milestone: group_milestone_1)
+              new_project.update(namespace: group_2)
+
+              group_2.add_users([user], GroupMember::DEVELOPER)
+            end
+
+            it 'assigns to new group milestone of same title' do
+              expect(new_issue.reload.project).to eq(new_project)
+              expect(new_issue.reload.milestone).to eq(group_milestone_2)
+            end
+          end
+        end
+      end
+
       context 'generic issue' do
         include_context 'issue move executed'
 
@@ -55,11 +109,6 @@ describe Issues::MoveService, services: true do
           expect(new_issue.project).to eq new_project
         end
 
-        it 'assigns milestone to new issue' do
-          expect(new_issue.reload.milestone.title).to eq 'v9.0'
-          expect(new_issue.reload.milestone).to eq(milestone2)
-        end
-
         it 'assign labels to new issue' do
           expected_label_titles = new_issue.reload.labels.map(&:title)
           expect(expected_label_titles).to include 'label1'