Remove the injection of `Gitlab::IssuableMetadata` in the API

With the BatchLoader implementation in the
`Gitlab::Entities::IssuableEntity` it was not necessary anymore to
sent this object around.

ee/lib/api/epics.rb

@@ -42,7 +42,7 @@ module API
       get ':id/(-/)epics' do
         epics = paginate(find_epics(finder_params: { group_id: user_group.id })).with_api_entity_associations
 
-        # issuable_metadata is the standard used by the Todo API
+        # issuable_metadata has to be set because `Entities::Epic` doesn't inherit from `Entities::IssuableEntity`
         extra_options = { issuable_metadata: Gitlab::IssuableMetadata.new(current_user, epics).data, with_labels_details: declared_params[:with_labels_details] }
         present epics, epic_options.merge(extra_options)
       end

lib/api/entities/issuable_entity.rb

@@ -14,19 +14,20 @@ module API
         super
       end
 
-      def issuable_metadata(user: nil)
-        # Because of the presence of the `user` parameter, we can't
-        # use the same lazy association.
-        return Gitlab::IssuableMetadata.new(user, [object]).data[object.id] if user
-
-        lazy_issuable_metadata
+      def issuable_metadata
+        options.dig(:issuable_metadata, object.id) || lazy_issuable_metadata
       end
 
       protected
 
+      # This method will preload the `issuable_metadata` for the current
+      # entity according to the current top-level entity options, such
+      # as the current_user.
       def lazy_issuable_metadata
-        BatchLoader.for(object).batch(key: :issuable_metadata) do |models, loader|
-          issuable_metadata = Gitlab::IssuableMetadata.new(nil, models)
+        BatchLoader.for(object).batch(key: [current_user, :issuable_metadata]) do |models, loader, args|
+          current_user = args[:key].first
+
+          issuable_metadata = Gitlab::IssuableMetadata.new(current_user, models)
           metadata_by_id = issuable_metadata.data
 
           models.each do |issuable|
@@ -34,6 +35,12 @@ module API
           end
         end
       end
+
+      private
+
+      def current_user
+        options[:current_user]
+      end
     end
   end
 end

lib/api/entities/issue_basic.rb

@@ -22,7 +22,7 @@ module API
       end
 
       expose(:user_notes_count)     { |issue, options| issuable_metadata.user_notes_count }
-      expose(:merge_requests_count) { |issue, options| issuable_metadata(user: options[:current_user]).merge_requests_count }
+      expose(:merge_requests_count) { |issue, options| issuable_metadata.merge_requests_count }
       expose(:upvotes)              { |issue, options| issuable_metadata.upvotes }
       expose(:downvotes)            { |issue, options| issuable_metadata.downvotes }
       expose :due_date

lib/api/entities/merge_request_basic.rb

@@ -26,14 +26,7 @@ module API
       expose(:upvotes)          { |merge_request, options| issuable_metadata.upvotes }
       expose(:downvotes)        { |merge_request, options| issuable_metadata.downvotes }
 
-      with_options using: Entities::UserBasic do
-        expose :author, as: :author
-        expose :assignees
-        expose :assignee do |merge_request, options|
-          merge_request.assignees.first
-        end
-      end
-
+      expose :author, :assignees, :assignee, using: Entities::UserBasic
       expose :source_project_id, :target_project_id
       expose :labels do |merge_request, options|
         if options[:with_labels_details]

lib/api/issues.rb

@@ -107,7 +107,6 @@ module API
           with: Entities::Issue,
           with_labels_details: declared_params[:with_labels_details],
           current_user: current_user,
-          issuable_metadata: Gitlab::IssuableMetadata.new(current_user, issues).data,
           include_subscribed: false
         }
 
@@ -133,7 +132,6 @@ module API
           with: Entities::Issue,
           with_labels_details: declared_params[:with_labels_details],
           current_user: current_user,
-          issuable_metadata: Gitlab::IssuableMetadata.new(current_user, issues).data,
           include_subscribed: false,
           group: user_group
         }
@@ -170,7 +168,6 @@ module API
           with_labels_details: declared_params[:with_labels_details],
           current_user: current_user,
           project: user_project,
-          issuable_metadata: Gitlab::IssuableMetadata.new(current_user, issues).data,
           include_subscribed: false
         }
 

lib/api/merge_requests.rb

@@ -93,7 +93,6 @@ module API
         if params[:view] == 'simple'
           options[:with] = Entities::MergeRequestSimple
         else
-          options[:issuable_metadata] = Gitlab::IssuableMetadata.new(current_user, merge_requests).data
           options[:skip_merge_status_recheck] = !declared_params[:with_merge_status_recheck]
         end