Optimize for setting MR assignees

This uses the new specific service from
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57523 to handle
cases where we are setting assignees.

app/services/merge_requests/update_assignees_service.rb

@@ -7,15 +7,17 @@ module MergeRequests
     # This saves a lot of queries for irrelevant things that cannot possibly
     # change in the execution of this service.
     def execute(merge_request)
-      return unless current_user&.can?(:update_merge_request, merge_request)
+      return merge_request unless current_user&.can?(:update_merge_request, merge_request)
 
       old_ids = merge_request.assignees.map(&:id)
-      return if old_ids.to_set == update_attrs[:assignee_ids].to_set # no-change
+      return merge_request if old_ids.to_set == update_attrs[:assignee_ids].to_set # no-change
 
       merge_request.update!(**update_attrs)
 
       # Defer the more expensive operations (handle_assignee_changes) to the background
       MergeRequests::AssigneesChangeWorker.perform_async(merge_request.id, current_user.id, old_ids)
+
+      merge_request
     end
 
     def handle_assignee_changes(merge_request, old_assignees)

lib/api/merge_requests.rb

@@ -424,7 +424,14 @@ module API
         mr_params[:force_remove_source_branch] = mr_params.delete(:remove_source_branch) if mr_params.has_key?(:remove_source_branch)
         mr_params = convert_parameters_from_legacy_format(mr_params)
 
-        merge_request = ::MergeRequests::UpdateService.new(user_project, current_user, mr_params).execute(merge_request)
+        service = if mr_params.keys == [:assignee_ids] || mr_params.keys == [:assignee_id]
+                    mr_params[:assignee_ids] = Array.wrap(mr_params[:assignee_ids]) if mr_params.key?(:assignee_id)
+                    ::MergeRequests::UpdateAssigneesService
+                  else
+                    ::MergeRequests::UpdateService
+                  end
+
+        merge_request = service.new(user_project, current_user, mr_params).execute(merge_request)
 
         handle_merge_request_errors!(merge_request)
 

spec/requests/api/merge_requests_spec.rb

@@ -2151,6 +2151,23 @@ RSpec.describe API::MergeRequests do
       let(:entity) { merge_request }
     end
 
+    context 'when only assignee_ids are provided' do
+      let(:params) do
+        {
+          assignee_ids: [user2.id]
+        }
+      end
+
+      it 'sets the assignees' do
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user), params: params
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response['assignees']).to contain_exactly(
+          a_hash_including('name' => user2.name)
+        )
+      end
+    end
+
     context 'accepts reviewer_ids' do
       let(:params) do
         {