Add from_fork scope

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/22418,
a change has been made that required the need to disable the
`CodeReuse/ActiveRecord` cop on `merge_requests_for_forks`.

To be able to enable it again, added `from_fork` scope and use
that instead.

app/models/merge_request.rb

@@ -263,8 +263,9 @@ class MergeRequest < ApplicationRecord
   scope :by_milestone, ->(milestone) { where(milestone_id: milestone) }
   scope :of_projects, ->(ids) { where(target_project_id: ids) }
   scope :from_project, ->(project) { where(source_project_id: project.id) }
+  scope :from_fork, -> { where('source_project_id <> target_project_id') }
   scope :from_and_to_forks, ->(project) do
-    where('source_project_id <> target_project_id AND (source_project_id = ? OR target_project_id = ?)', project.id, project.id)
+    from_fork.where('source_project_id = ? OR target_project_id = ?', project.id, project.id)
   end
   scope :merged, -> { with_state(:merged) }
   scope :closed_and_merged, -> { with_states(:closed, :merged) }

app/services/merge_requests/refresh_service.rb

@@ -294,14 +294,14 @@ module MergeRequests
       @source_merge_requests ||= merge_requests_for(@push.branch_name)
     end
 
-    # rubocop: disable CodeReuse/ActiveRecord
     def merge_requests_for_forks
       @merge_requests_for_forks ||=
-        MergeRequest.opened
-          .where(source_branch: @push.branch_name, source_project: @project)
-          .where.not(target_project: @project)
+        MergeRequest
+          .opened
+          .from_project(project)
+          .from_source_branches(@push.branch_name)
+          .from_fork
     end
-    # rubocop: enable CodeReuse/ActiveRecord
   end
 end
 

spec/models/merge_request_spec.rb

@@ -4951,4 +4951,15 @@ RSpec.describe MergeRequest, factory_default: :keep do
       it { is_expected.to eq(true) }
     end
   end
+
+  describe '.from_fork' do
+    let!(:project) { create(:project, :repository) }
+    let!(:forked_project) { fork_project(project) }
+    let!(:fork_mr) { create(:merge_request, source_project: forked_project, target_project: project) }
+    let!(:regular_mr) { create(:merge_request, source_project: project) }
+
+    it 'returns merge requests from forks only' do
+      expect(described_class.from_fork).to eq([fork_mr])
+    end
+  end
 end