Fixes milestone/merge_request API endpoint to really scope the results

The MergeRequestsFinder uses the milestone_title instead of the
milestone_id

app/finders/issues_finder.rb

@@ -9,7 +9,7 @@
 #     state: 'open' or 'closed' or 'all'
 #     group_id: integer
 #     project_id: integer
-#     milestone_id: integer
+#     milestone_title: string
 #     assignee_id: integer
 #     search: string
 #     label_name: string

app/finders/merge_requests_finder.rb

@@ -9,7 +9,7 @@
 #     state: 'open' or 'closed' or 'all'
 #     group_id: integer
 #     project_id: integer
-#     milestone_id: integer
+#     milestone_title: string
 #     assignee_id: integer
 #     search: string
 #     label_name: string

changelogs/unreleased/bug-api_milestone_merge_requests_scope.yml

+---
+title: Fixes milestone/merge_requests endpoint to actually scope the result
+merge_request:
+author: Joren De Groof

lib/api/milestones.rb

@@ -139,7 +139,7 @@ module API
         finder_params = {
           project_id: user_project.id,
-          milestone_id: milestone.id,
+          milestone_title: milestone.title,
           sort: 'position_asc'
         }

spec/requests/api/milestones_spec.rb

@@ -306,6 +306,8 @@ describe API::Milestones, api: true  do
     end
     it 'returns project merge_requests for a particular milestone' do
+      # eager-load another_merge_request
+      another_merge_request
       get api("/projects/#{project.id}/milestones/#{milestone.id}/merge_requests", user)
       expect(response).to have_http_status(200)