Fix performance problem of accessing tag list for projects api endpoints

changelogs/unreleased/fix-performance-problem-of-tags-query.yml

+---
+title: Fix performance problem of accessing tag list for projects api endpoints
+merge_request:
+author:
+type: performance

lib/api/entities.rb

@@ -135,10 +135,13 @@ module API
       expose :custom_attributes, using: 'API::Entities::CustomAttribute', if: :with_custom_attributes
 
       def self.preload_relation(projects_relation, options =  {})
+        # Preloading tags, should be done with using only `:tags`,
+        # as `:tags` are defined as: `has_many :tags, through: :taggings`
+        # N+1 is solved then by using `subject.tags.map(&:name)`
+        # MR describing the solution: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20555
         projects_relation.preload(:project_feature, :route)
-                         .preload(:import_state)
-                         .preload(namespace: [:route, :owner],
-                                  tags: :taggings)
+                         .preload(:import_state, :tags)
+                         .preload(namespace: [:route, :owner])
       end
     end
 
@@ -212,11 +215,15 @@ module API
       expose :statistics, using: 'API::Entities::ProjectStatistics', if: :statistics
 
       def self.preload_relation(projects_relation, options =  {})
+        # Preloading tags, should be done with using only `:tags`,
+        # as `:tags` are defined as: `has_many :tags, through: :taggings`
+        # N+1 is solved then by using `subject.tags.map(&:name)`
+        # MR describing the solution: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20555
         super(projects_relation).preload(:group)
                                 .preload(project_group_links: :group,
                                          fork_network: :root_project,
                                          forked_project_link: :forked_from_project,
-                                         forked_from_project: [:route, :forks, namespace: :route, tags: :taggings])
+                                         forked_from_project: [:route, :forks, :tags, namespace: :route])
       end
 
       def self.forks_counting_projects(projects_relation)