add related merge request endpoint

changelogs/unreleased/related_mrs.yml

+---
+title: Add API endpoint to list issue related merge requests
+merge_request: 21806
+author: Helmut Januschka
+type: added

doc/api/issues.md

@@ -1113,6 +1113,93 @@ Example response:
 }
 ```
 
+## List merge requests related to issue
+
+Get all the merge requests that are related to the issue.
+
+```
+GET /projects/:id/issues/:issue_id/related_merge_requests
+```
+
+| Attribute   | Type    | Required | Description                          |
+|-------------|---------|----------|--------------------------------------|
+| `id`        | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user  |
+| `issue_iid` | integer | yes      | The internal ID of a project's issue |
+
+```sh
+curl --request GET --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/projects/1/issues/11/related_merge_requests
+```
+
+Example response:
+
+```json
+[
+  {
+    "id": 29,
+    "iid": 11,
+    "project_id": 1,
+    "title": "Provident eius eos blanditiis consequatur neque odit.",
+    "description": "Ut consequatur ipsa aspernatur quisquam voluptatum fugit. Qui harum corporis quo fuga ut incidunt veritatis. Autem necessitatibus et harum occaecati nihil ea.\r\n\r\ntwitter/flight#8",
+    "state": "opened",
+    "created_at": "2018-09-18T14:36:15.510Z",
+    "updated_at": "2018-09-19T07:45:13.089Z",
+    "target_branch": "v2.x",
+    "source_branch": "so_long_jquery",
+    "upvotes": 0,
+    "downvotes": 0,
+    "author": {
+      "id": 14,
+      "name": "Verna Hills",
+      "username": "lawanda_reinger",
+      "state": "active",
+      "avatar_url": "https://www.gravatar.com/avatar/de68a91aeab1cff563795fb98a0c2cc0?s=80&d=identicon",
+      "web_url": "https://gitlab.example.com/lawanda_reinger"
+    },
+    "assignee": {
+      "id": 19,
+      "name": "Jody Baumbach",
+      "username": "felipa.kuvalis",
+      "state": "active",
+      "avatar_url": "https://www.gravatar.com/avatar/6541fc75fc4e87e203529bd275fafd07?s=80&d=identicon",
+      "web_url": "https://gitlab.example.com/felipa.kuvalis"
+    },
+    "source_project_id": 1,
+    "target_project_id": 1,
+    "labels": [],
+    "work_in_progress": false,
+    "milestone": {
+      "id": 27,
+      "iid": 2,
+      "project_id": 1,
+      "title": "v1.0",
+      "description": "Et tenetur voluptatem minima doloribus vero dignissimos vitae.",
+      "state": "active",
+      "created_at": "2018-09-18T14:35:44.353Z",
+      "updated_at": "2018-09-18T14:35:44.353Z",
+      "due_date": null,
+      "start_date": null,
+      "web_url": "https://gitlab.example.com/twitter/flight/milestones/2"
+    },
+    "merge_when_pipeline_succeeds": false,
+    "merge_status": "cannot_be_merged",
+    "sha": "3b7b528e9353295c1c125dad281ac5b5deae5f12",
+    "merge_commit_sha": null,
+    "user_notes_count": 9,
+    "discussion_locked": null,
+    "should_remove_source_branch": null,
+    "force_remove_source_branch": false,
+    "web_url": "https://gitlab.example.com/twitter/flight/merge_requests/4",
+    "time_stats": {
+      "time_estimate": 0,
+      "total_time_spent": 0,
+      "human_time_estimate": null,
+      "human_total_time_spent": null
+    },
+    "squash": false
+  }
+]
+```
+
 ## List merge requests that will close issue on merge
 
 Get all the merge requests that will close issue when merged.

lib/api/issues.rb

@@ -294,6 +294,30 @@ module API
       end
       # rubocop: enable CodeReuse/ActiveRecord
 
+      desc 'List merge requests that are related to the issue'  do
+        success Entities::MergeRequestBasic
+      end
+      params do
+        requires :issue_iid, type: Integer, desc: 'The internal ID of a project issue'
+      end
+      get ':id/issues/:issue_iid/related_merge_requests' do
+        issue = find_project_issue(params[:issue_iid])
+
+        merge_request_iids = ::Issues::ReferencedMergeRequestsService.new(user_project, current_user)
+          .execute(issue)
+          .flatten
+          .map(&:iid)
+
+        merge_requests =
+          if merge_request_iids.present?
+            MergeRequestsFinder.new(current_user, project_id: user_project.id, iids: merge_request_iids).execute
+          else
+            MergeRequest.none
+          end
+
+        present paginate(merge_requests), with: Entities::MergeRequestBasic, current_user: current_user, project: user_project
+      end
+
       desc 'List merge requests closing issue'  do
         success Entities::MergeRequestBasic
       end

spec/requests/api/issues_spec.rb

@@ -1801,6 +1801,74 @@ describe API::Issues do
     end
   end
 
+  describe 'GET :id/issues/:issue_iid/related_merge_requests' do
+    def get_related_merge_requests(project_id, issue_iid, user = nil)
+      get api("/projects/#{project_id}/issues/#{issue_iid}/related_merge_requests", user)
+    end
+
+    def create_referencing_mr(user, project, issue)
+      attributes = {
+        author: user,
+        source_project: project,
+        target_project: project,
+        source_branch: "master",
+        target_branch: "test",
+        description: "See #{issue.to_reference}"
+      }
+      create(:merge_request, attributes).tap do |merge_request|
+        create(:note, :system, project: project, noteable: issue, author: user, note: merge_request.to_reference(full: true))
+      end
+    end
+
+    let!(:related_mr) { create_referencing_mr(user, project, issue) }
+
+    context 'when unauthenticated' do
+      it 'return list of referenced merge requests from issue' do
+        get_related_merge_requests(project.id, issue.iid)
+
+        expect_paginated_array_response(size: 1)
+      end
+
+      it 'renders 404 if project is not visible' do
+        private_project = create(:project, :private)
+        private_issue = create(:issue, project: private_project)
+        create_referencing_mr(user, private_project, private_issue)
+
+        get_related_merge_requests(private_project.id, private_issue.iid)
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+    end
+
+    it 'returns merge requests that mentioned a issue' do
+      create(:merge_request,
+            :simple,
+            author: user,
+            source_project: project,
+            target_project: project,
+            description: "Some description")
+
+      get_related_merge_requests(project.id, issue.iid, user)
+
+      expect_paginated_array_response(size: 1)
+      expect(json_response.first['id']).to eq(related_mr.id)
+    end
+
+    context 'no merge request mentioned a issue' do
+      it 'returns empty array' do
+        get_related_merge_requests(project.id, closed_issue.iid, user)
+
+        expect_paginated_array_response(size: 0)
+      end
+    end
+
+    it "returns 404 when issue doesn't exists" do
+      get_related_merge_requests(project.id, 999999, user)
+
+      expect(response).to have_gitlab_http_status(404)
+    end
+  end
+
   describe "GET /projects/:id/issues/:issue_iid/user_agent_detail" do
     let!(:user_agent_detail) { create(:user_agent_detail, subject: issue) }