Batch load merge requests count to avoid N+1

Create a resolver to batch load merge requests count for issues.

app/graphql/resolvers/merge_requests_count_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  class MergeRequestsCountResolver < BaseResolver
+    include Gitlab::Graphql::Authorize::AuthorizeResource
+
+    type GraphQL::Types::Int, null: true
+
+    def resolve
+      authorize!(object)
+
+      BatchLoader::GraphQL.for(object.id).batch do |ids, loader, args|
+        counts = MergeRequestsClosingIssues.count_for_collection(ids, context[:current_user]).to_h
+
+        ids.each do |id|
+          loader.call(id, counts[id] || 0)
+        end
+      end
+    end
+
+    def authorized_resource?(object)
+      ability = "read_#{object.class.name.underscore}".to_sym
+      context[:current_user].present? && Ability.allowed?(context[:current_user], ability, object)
+    end
+  end
+end

app/graphql/types/issue_type.rb

@@ -60,8 +60,9 @@ module Types
           description: 'Number of upvotes the issue has received.'
     field :downvotes, GraphQL::Types::Int, null: false,
           description: 'Number of downvotes the issue has received.'
-    field :merge_requests_count, GraphQL::INT_TYPE, null: false,
-          description: 'Number of merge requests that close the issue on merge.'
+    field :merge_requests_count, GraphQL::Types::Int, null: false,
+          description: 'Number of merge requests that close the issue on merge.',
+          resolver: Resolvers::MergeRequestsCountResolver
     field :user_notes_count, GraphQL::Types::Int, null: false,
           description: 'Number of user notes of the issue.',
           resolver: Resolvers::UserNotesCountResolver

spec/graphql/resolvers/merge_requests_count_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::MergeRequestsCountResolver do
+  include GraphqlHelpers
+
+  describe '#resolve' do
+    let_it_be(:user) { create(:user) }
+
+    specify do
+      expect(described_class).to have_nullable_graphql_type(GraphQL::Types::Int)
+    end
+
+    context 'when counting closing merge requests from a public issue' do
+      let_it_be(:project) { create(:project, :repository, :public) }
+      let_it_be(:issue) { create(:issue, project: project) }
+      let_it_be(:merge_request) { create(:merge_requests_closing_issues, issue: issue) }
+
+      subject { batch_sync { resolve_merge_requests_count(issue) } }
+
+      it 'returns the count of the merge requests closing the issue' do
+        expect(subject).to eq(1)
+      end
+    end
+
+    context 'when attempting to view a private issue' do
+      let_it_be(:private_project) { create(:project, :repository, :private) }
+      let_it_be(:issue) { create(:issue, project: private_project) }
+
+      before_all do
+        create(:merge_requests_closing_issues, issue: issue)
+        create(:merge_requests_closing_issues, issue: issue)
+      end
+
+      context 'when a user has permission to view the issue' do
+        before do
+          private_project.add_developer(user)
+        end
+
+        subject { batch_sync { resolve_merge_requests_count(issue) } }
+
+        it 'returns the count of the merge requests closing the issue' do
+          expect(subject).to eq(2)
+        end
+      end
+
+      context 'when a user does not have permission to view the issue' do
+        subject { batch_sync { resolve_merge_requests_count(issue) } }
+
+        it 'raises an error' do
+          expect { subject }.to raise_error(Gitlab::Graphql::Errors::ResourceNotAvailable)
+        end
+      end
+    end
+  end
+
+  def resolve_merge_requests_count(obj)
+    resolve(described_class, obj: obj, ctx: { current_user: user })
+  end
+end