Fix MergeRequest's MergeUser for non-MWPS merges in GraphQL API

Field MergeUser used to always be nil for merges done without
Merge When Pipeline Succeeds function, despite documentation
stating that it returns 'User who merged this merge request'.
I change it so that it works for MR without MWPS.
I've also clarified that mergeUser will be returned as soon
as MWPS is set, even before the merge actually happens.

Changelog: fixed

app/graphql/types/merge_request_type.rb

@@ -196,7 +196,7 @@ module Types
     field :auto_merge_strategy, GraphQL::Types::String, null: true,
           description: 'Selected auto merge strategy.'
     field :merge_user, Types::UserType, null: true,
-          description: 'User who merged this merge request.'
+          description: 'User who merged this merge request or set it to merge when pipeline succeeds.'
     field :timelogs, Types::TimelogType.connection_type, null: false,
           description: 'Timelogs on the merge request.'
 
@@ -268,6 +268,10 @@ module Types
     def reviewers
       object.reviewers
     end
+
+    def merge_user
+      object.metrics&.merged_by || object.merge_user
+    end
   end
 end
 

doc/api/graphql/reference/index.md

@@ -11698,7 +11698,7 @@ Maven metadata.
 | <a id="mergerequestmergestatus"></a>`mergeStatus` **{warning-solid}** | [`String`](#string) | **Deprecated** in 14.0. This was renamed. Use: [`MergeRequest.mergeStatusEnum`](#mergerequestmergestatusenum). |
 | <a id="mergerequestmergestatusenum"></a>`mergeStatusEnum` | [`MergeStatus`](#mergestatus) | Merge status of the merge request. |
 | <a id="mergerequestmergetrainscount"></a>`mergeTrainsCount` | [`Int`](#int) | Number of merge requests in the merge train. |
-| <a id="mergerequestmergeuser"></a>`mergeUser` | [`UserCore`](#usercore) | User who merged this merge request. |
+| <a id="mergerequestmergeuser"></a>`mergeUser` | [`UserCore`](#usercore) | User who merged this merge request or set it to merge when pipeline succeeds. |
 | <a id="mergerequestmergewhenpipelinesucceeds"></a>`mergeWhenPipelineSucceeds` | [`Boolean`](#boolean) | Indicates if the merge has been set to be merged when its pipeline succeeds (MWPS). |
 | <a id="mergerequestmergeable"></a>`mergeable` | [`Boolean!`](#boolean) | Indicates if the merge request is mergeable. |
 | <a id="mergerequestmergeablediscussionsstate"></a>`mergeableDiscussionsState` | [`Boolean`](#boolean) | Indicates if all discussions in the merge request have been resolved, allowing the merge request to be merged. |

spec/graphql/types/merge_request_type_spec.rb

@@ -133,4 +133,28 @@ RSpec.describe GitlabSchema.types['MergeRequest'] do
       end
     end
   end
+
+  describe '#merge_user' do
+    let_it_be(:project) { create(:project, :public) }
+
+    context 'when MR is merged' do
+      let(:merge_request) { create(:merge_request, :with_merged_metrics, target_project: project, source_project: project) }
+
+      it 'is not nil' do
+        value = resolve_field(:merge_user, merge_request)
+
+        expect(value).not_to be_nil
+      end
+    end
+
+    context 'when MR is set to merge when pipeline succeeds' do
+      let(:merge_request) { create(:merge_request, :merge_when_pipeline_succeeds, target_project: project, source_project: project) }
+
+      it 'is not nil' do
+        value = resolve_field(:merge_user, merge_request)
+
+        expect(value).not_to be_nil
+      end
+    end
+  end
 end