Eliminate N+1 queries in /api/groups/:id

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15475/diffs, a
significant amount of work went into eliminating N+1 queries in the
/api/groups/:id/projects endpoint. We can reuse the
`Entities::Project.prepare_relation` call on the projects.

In a group with 2,573 projects on GitLab.com, this change significantly
improves performance:

* 18019 SQL queries down to 21
* Time spent in DB: 70 s down to 384 ms

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49845

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 97b421e8651..e0a48908122 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -344,19 +344,23 @@ module API
 
     class GroupDetail < Group
       expose :projects, using: Entities::Project do |group, options|
-        GroupProjectsFinder.new(
+        projects = GroupProjectsFinder.new(
           group: group,
           current_user: options[:current_user],
           options: { only_owned: true }
         ).execute
+
+        Entities::Project.prepare_relation(projects)
       end
 
       expose :shared_projects, using: Entities::Project do |group, options|
-        GroupProjectsFinder.new(
+        projects = GroupProjectsFinder.new(
           group: group,
           current_user: options[:current_user],
           options: { only_shared: true }
         ).execute
+
+        Entities::Project.prepare_relation(projects)
       end
     end