Fixes GraphQl merge request types commits count

Fixes an issue where sometimes the commits count
would return `null` instead of the correct value.

app/graphql/types/merge_request_type.rb

@@ -152,7 +152,7 @@ module Types
     end
     field :task_completion_status, Types::TaskCompletionStatus, null: false,
           description: Types::TaskCompletionStatus.description
-    field :commit_count, GraphQL::INT_TYPE, null: true,
+    field :commit_count, GraphQL::INT_TYPE, null: true, method: :commits_count,
           description: 'Number of commits in the merge request'
     field :conflicts, GraphQL::BOOLEAN_TYPE, null: false, method: :cannot_be_merged?,
           description: 'Indicates if the merge request has conflicts'
@@ -218,10 +218,6 @@ module Types
       BatchLoaders::MergeRequestDiffSummaryBatchLoader.load_for(object)
     end
 
-    def commit_count
-      object&.metrics&.commits_count
-    end
-
     def source_branch_protected
       object.source_project.present? && ProtectedBranch.protected?(object.source_project, object.source_branch)
     end

changelogs/unreleased/ph-fixMRGraphQlCommitsCount.yml

+---
+title: Fixes GraphQl merge request types commits count
+merge_request: 52218
+author:
+type: fixed

spec/requests/api/graphql/project/merge_requests_spec.rb

@@ -196,17 +196,18 @@ RSpec.describe 'getting merge request listings nested in a project' do
     end
 
     context 'when requesting `commit_count`' do
-      let(:requested_fields) { [:commit_count] }
+      let(:merge_request_with_commits) { create(:merge_request, source_project: project) }
+      let(:search_params) { { iids: [merge_request_a.iid.to_s, merge_request_with_commits.iid.to_s] } }
+      let(:requested_fields) { [:iid, :commit_count] }
 
       it 'exposes `commit_count`' do
-        merge_request_a.metrics.update!(commits_count: 5)
-
         execute_query
 
-        expect(results).to include(a_hash_including('commitCount' => 5))
+        expect(results).to match_array([
+          { "iid" => merge_request_a.iid.to_s, "commitCount" => 0 },
+          { "iid" => merge_request_with_commits.iid.to_s, "commitCount" => 29 }
+        ])
       end
-
-      include_examples 'N+1 query check'
     end
 
     context 'when requesting `merged_at`' do