Refactor AutocompleteControllerdefine-abstraction-levels

This refactors the AutocompleteController according to the guidelines
and boundaries discussed in
https://gitlab.com/gitlab-org/gitlab-ce/issues/49653. Specifically,
ActiveRecord logic is moved to different finders, which are then used in
the controller. View logic in turn is moved to presenters, instead of
directly using ActiveRecord's "to_json" method.

The finder MoveToProjectFinder is also adjusted according to the
abstraction guidelines and boundaries, resulting in a much more simple
finder.

By using finders (and other abstractions) more actively, we can push a
lot of logic out of the controller. We also remove the need for various
"before_action" hooks, though this could be achieved without using
finders as well.

The various finders related to AutcompleteController have also been
moved into a namespace. This removes the need for calling everything
"AutocompleteSmurfFinder", instead you can use
"Autocomplete::SmurfFinder".

4 files changed, 192 insertions, 0 deletions

diff --git a/app/finders/autocomplete/group_finder.rb b/app/finders/autocomplete/group_finder.rb
new file mode 100644
index 00000000000..dd97ac4c817
--- /dev/null
+++ b/app/finders/autocomplete/group_finder.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module Autocomplete
+  # Finder for retrieving a group to use for autocomplete data sources.
+  class GroupFinder
+    attr_reader :current_user, :project, :group_id
+
+    # current_user - The currently logged in user, if any.
+    # project - The Project (if any) to use for the autocomplete data sources.
+    # params - A Hash containing parameters to use for finding the project.
+    #
+    # The following parameters are supported:
+    #
+    # * group_id: The ID of the group to find.
+    def initialize(current_user = nil, project = nil, params = {})
+      @current_user = current_user
+      @project = project
+      @group_id = params[:group_id]
+    end
+
+    # Attempts to find a Group based on the current group ID.
+    def execute
+      return unless project.blank? && group_id.present?
+
+      group = Group.find(group_id)
+
+      # This removes the need for using `return render_404` and similar patterns
+      # in controllers that use this finder.
+      unless Ability.allowed?(current_user, :read_group, group)
+        raise ActiveRecord::RecordNotFound
+          .new("Could not find a Group with ID #{group_id}")
+      end
+
+      group
+    end
+  end
+end
diff --git a/app/finders/autocomplete/move_to_project_finder.rb b/app/finders/autocomplete/move_to_project_finder.rb
new file mode 100644
index 00000000000..edaf74c5f92
--- /dev/null
+++ b/app/finders/autocomplete/move_to_project_finder.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module Autocomplete
+  # Finder that retrieves a list of projects that an issue can be moved to.
+  class MoveToProjectFinder
+    attr_reader :current_user, :search, :project_id, :offset_id
+
+    # current_user - The User object of the user that wants to view the list of
+    #                projects.
+    #
+    # params - A Hash containing additional parameters to set.
+    #
+    # The following parameters can be set (as Symbols):
+    #
+    # * search: An optional search query to apply to the list of projects.
+    # * project_id: The ID of a project to exclude from the returned relation.
+    # * offset_id: The ID of a project to use for pagination. When given, only
+    #   projects with a lower ID are included in the list.
+    def initialize(current_user, params = {})
+      @current_user = current_user
+      @search = params[:search]
+      @project_id = params[:project_id]
+      @offset_id = params[:offset_id]
+    end
+
+    def execute
+      current_user
+        .projects_where_can_admin_issues
+        .optionally_search(search)
+        .excluding_project(project_id)
+        .paginate_in_descending_order_using_id(before: offset_id)
+        .eager_load_namespace_and_owner
+    end
+  end
+end
diff --git a/app/finders/autocomplete/project_finder.rb b/app/finders/autocomplete/project_finder.rb
new file mode 100644
index 00000000000..3a4696f4c2e
--- /dev/null
+++ b/app/finders/autocomplete/project_finder.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module Autocomplete
+  # Finder for retrieving a project to use for autocomplete data sources.
+  class ProjectFinder
+    attr_reader :current_user, :project_id
+
+    # current_user - The currently logged in user, if any.
+    # params - A Hash containing parameters to use for finding the project.
+    #
+    # The following parameters are supported:
+    #
+    # * project_id: The ID of the project to find.
+    def initialize(current_user = nil, params = {})
+      @current_user = current_user
+      @project_id = params[:project_id]
+    end
+
+    # Attempts to find a Project based on the current project ID.
+    def execute
+      return if project_id.blank?
+
+      project = Project.find(project_id)
+
+      # This removes the need for using `return render_404` and similar patterns
+      # in controllers that use this finder.
+      unless Ability.allowed?(current_user, :read_project, project)
+        raise ActiveRecord::RecordNotFound
+          .new("Could not find a Project with ID #{project_id}")
+      end
+
+      project
+    end
+  end
+end
diff --git a/app/finders/autocomplete/users_finder.rb b/app/finders/autocomplete/users_finder.rb
new file mode 100644
index 00000000000..b2557469079
--- /dev/null
+++ b/app/finders/autocomplete/users_finder.rb
@@ -0,0 +1,85 @@
+# frozen_string_literal: true
+
+module Autocomplete
+  class UsersFinder
+    # The number of users to display in the results is hardcoded to 20, and
+    # pagination is not supported. This ensures that performance remains
+    # consistent and removes the need for implementing keyset pagination to
+    # ensure good performance.
+    LIMIT = 20
+
+    attr_reader :current_user, :project, :group, :search, :skip_users,
+                :author_id, :todo_filter, :todo_state_filter,
+                :filter_by_current_user
+
+    def initialize(params:, current_user:, project:, group:)
+      @current_user = current_user
+      @project = project
+      @group = group
+      @search = params[:search]
+      @skip_users = params[:skip_users]
+      @author_id = params[:author_id]
+      @todo_filter = params[:todo_filter]
+      @todo_state_filter = params[:todo_state_filter]
+      @filter_by_current_user = params[:current_user]
+    end
+
+    def execute
+      items = limited_users
+
+      if search.blank?
+        # Include current user if available to filter by "Me"
+        items.unshift(current_user) if prepend_current_user?
+
+        if prepend_author? && (author = User.find_by_id(author_id))
+          items.unshift(author)
+        end
+      end
+
+      items.uniq
+    end
+
+    private
+
+    # Returns the users based on the input parameters, as an Array.
+    #
+    # This method is separate so it is easier to extend in EE.
+    def limited_users
+      # When changing the order of these method calls, make sure that
+      # reorder_by_name() is called _before_ optionally_search(), otherwise
+      # reorder_by_name will break the ORDER BY applied in optionally_search().
+      find_users
+        .active
+        .reorder_by_name
+        .optionally_search(search)
+        .where_not_in(skip_users)
+        .limit_to_todo_authors(
+          user: current_user,
+          with_todos: todo_filter,
+          todo_state: todo_state_filter
+        )
+        .limit(LIMIT)
+        .to_a
+    end
+
+    def prepend_current_user?
+      filter_by_current_user.present? && current_user
+    end
+
+    def prepend_author?
+      author_id.present? && current_user
+    end
+
+    def find_users
+      if project
+        project.authorized_users.union_with_user(author_id)
+      elsif group
+        group.users_with_parents
+      elsif current_user
+        User.all
+      else
+        User.none
+      end
+    end
+  end
+end