Further reduce N+1 on Jira pull endpoints

This builds on the optimisations in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57482 to further
reduce N+1s during loading source and target project associations.

The N+1 issues happen when a collection of merge requests are serialised
as `API::Github::Entities::PullRequest` entities.

This change also removes duplicate `preload` logic which was made
redundant after 7856804b.

app/models/merge_request.rb

@@ -316,7 +316,7 @@ class MergeRequest < ApplicationRecord
   }
 
   scope :with_csv_entity_associations, -> { preload(:assignees, :approved_by_users, :author, :milestone, metrics: [:merged_by]) }
-  scope :with_jira_integration_associations, -> { preload(:metrics, :assignees, :author, :target_project, :source_project) }
+  scope :with_jira_integration_associations, -> { preload_routables.preload(:metrics, :assignees, :author) }
 
   scope :by_target_branch_wildcard, ->(wildcard_branch_name) do
     where("target_branch LIKE ?", ApplicationRecord.sanitize_sql_like(wildcard_branch_name).tr('*', '%'))

changelogs/unreleased/33741-further-optimise-api-v3-github-endpoints.yml

+---
+title: Resolve more N+1 issues in Jira pulls API
+merge_request: 57658
+author:
+type: performance

lib/api/v3/github.rb

@@ -197,16 +197,13 @@ module API
 
         # Self-hosted Jira (tested on 7.11.1) requests this endpoint right
         # after fetching branches.
-        # rubocop: disable CodeReuse/ActiveRecord
         get ':namespace/:project/events' do
           user_project = find_project_with_access(params)
 
           merge_requests = authorized_merge_requests_for_project(user_project)
-          merge_requests = merge_requests.preload(:author, :assignees, :metrics, source_project: :namespace, target_project: :namespace)
 
           present paginate(merge_requests), with: ::API::Github::Entities::PullRequestEvent
         end
-        # rubocop: enable CodeReuse/ActiveRecord
 
         params do
           use :project_full_path

spec/requests/api/v3/github_spec.rb

@@ -246,7 +246,10 @@ RSpec.describe API::V3::Github do
 
         control_count = ActiveRecord::QueryRecorder.new { perform_request }.count
 
-        create(:merge_request, source_project: project, source_branch: 'fix')
+        project3 = create(:project, :repository, creator: user)
+        project3.add_maintainer(user)
+        assignee3 = create(:user)
+        create(:merge_request, source_project: project3, target_project: project3, author: user, assignees: [assignee3])
 
         expect { perform_request }.not_to exceed_query_limit(control_count)
       end