5 files changed, 73 insertions, 1 deletions
diff --git a/app/models/issue.rb b/app/models/issue.rb
index b19d82a322a..c8475d21eb5 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -39,6 +39,8 @@ class Issue < ActiveRecord::Base
 
   scope :created_after, -> (datetime) { where("created_at >= ?", datetime) }
 
+  scope :include_associations, -> { includes(:assignee, :labels, project: :namespace) }
+
   attr_spammable :title, spam_title: true
   attr_spammable :description, spam_description: true
 
diff --git a/app/views/shared/milestones/_tabs.html.haml b/app/views/shared/milestones/_tabs.html.haml
index 2b6ce2d7e7a..c8f2319d95a 100644
--- a/app/views/shared/milestones/_tabs.html.haml
+++ b/app/views/shared/milestones/_tabs.html.haml
@@ -21,7 +21,7 @@
 
 .tab-content.milestone-content
   .tab-pane.active#tab-issues
-    = render 'shared/milestones/issues_tab', issues: milestone.issues_visible_to_user(current_user), show_project_name: show_project_name, show_full_project_name: show_full_project_name
+    = render 'shared/milestones/issues_tab', issues: milestone.issues_visible_to_user(current_user).include_associations, show_project_name: show_project_name, show_full_project_name: show_full_project_name
   .tab-pane#tab-merge-requests
     = render 'shared/milestones/merge_requests_tab', merge_requests: milestone.merge_requests, show_project_name: show_project_name, show_full_project_name: show_full_project_name
   .tab-pane#tab-participants
diff --git a/changelogs/unreleased/jej-fix-n-1-queries-milestones-show.yml b/changelogs/unreleased/jej-fix-n-1-queries-milestones-show.yml
new file mode 100644
index 00000000000..ad6eba3faf2
--- /dev/null
+++ b/changelogs/unreleased/jej-fix-n-1-queries-milestones-show.yml
@@ -0,0 +1,4 @@
+---
+title: Fix N+1 queries on milestone show pages
+merge_request: 8185
+author: 
diff --git a/spec/features/milestones/show_spec.rb b/spec/features/milestones/show_spec.rb
new file mode 100644
index 00000000000..40b4dc63697
--- /dev/null
+++ b/spec/features/milestones/show_spec.rb
@@ -0,0 +1,26 @@
+require 'rails_helper'
+
+describe 'Milestone show', feature: true do
+  let(:user) { create(:user) }
+  let(:project) { create(:empty_project) }
+  let(:milestone) { create(:milestone, project: project) }
+  let(:labels) { create_list(:label, 2, project: project) }
+  let(:issue_params) { { project: project, assignee: user, author: user, milestone: milestone, labels: labels } }
+
+  before do
+    project.add_user(user, :developer) 
+    login_as(user)
+  end
+
+  def visit_milestone
+    visit namespace_project_milestone_path(project.namespace, project, milestone)
+  end
+
+  it 'avoids N+1 database queries' do
+    create(:labeled_issue, issue_params)
+    control_count = ActiveRecord::QueryRecorder.new { visit_milestone }.count
+    create_list(:labeled_issue, 10, issue_params)
+
+    expect { visit_milestone }.not_to exceed_query_limit(control_count)
+  end
+end
diff --git a/spec/support/query_recorder.rb b/spec/support/query_recorder.rb
new file mode 100644
index 00000000000..e40d5ebd9a8
--- /dev/null
+++ b/spec/support/query_recorder.rb
@@ -0,0 +1,40 @@
+module ActiveRecord
+  class QueryRecorder
+    attr_reader :log
+
+    def initialize(&block)
+      @log = []
+      ActiveSupport::Notifications.subscribed(method(:callback), 'sql.active_record', &block)
+    end
+
+    def callback(name, start, finish, message_id, values)
+      return if %w(CACHE SCHEMA).include?(values[:name])
+      @log << values[:sql]
+    end
+
+    def count
+      @log.count
+    end
+
+    def log_message
+      @log.join("\n\n")
+    end
+  end
+end
+
+RSpec::Matchers.define :exceed_query_limit do |expected|
+  supports_block_expectations
+
+  match do |block|
+    query_count(&block) > expected
+  end
+
+  failure_message_when_negated do |actual|
+    "Expected a maximum of #{expected} queries, got #{@recorder.count}:\n\n#{@recorder.log_message}"
+  end
+
+  def query_count(&block)
+    @recorder = ActiveRecord::QueryRecorder.new(&block)
+    @recorder.count
+  end
+end