Fix merge request GraphQL resolver

The resolver doesn't respond to pagination params such as `first`
Additionally, the resolver returns merge requests that belong to
all projects.

* update resolver to accept pagination params e.g `first`
* return only merge requests that belong to a project

app/graphql/resolvers/merge_requests_resolver.rb

@@ -20,8 +20,17 @@ module Resolvers
 
       args[:iids] ||= [args[:iid]].compact
 
-      args[:iids].map { |iid| batch_load(iid) }
-        .select(&:itself) # .compact doesn't work on BatchLoader
+      if args[:iids].any?
+        batch_load_merge_requests(args[:iids])
+      else
+        args[:project_id] = project.id
+
+        MergeRequestsFinder.new(context[:current_user], args).execute
+      end
+    end
+
+    def batch_load_merge_requests(iids)
+      iids.map { |iid| batch_load(iid) }.select(&:itself) # .compact doesn't work on BatchLoader
     end
 
     # rubocop: disable CodeReuse/ActiveRecord

changelogs/unreleased/34527-fix-graphql-endpoint-for-merge-requests.yml

+---
+title: Fix pagination in Merge Request GraphQL api
+merge_request: 28667
+author: briankabiro
+type: fixed

spec/requests/api/graphql/project/merge_request/diff_notes_spec.rb

@@ -5,7 +5,7 @@ require 'spec_helper'
 describe 'getting notes for a merge request' do
   include GraphqlHelpers
 
-  let(:noteable) { create(:merge_request) }
+  let_it_be(:noteable) { create(:merge_request) }
 
   def noteable_query(noteable_fields)
     <<~QRY

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

@@ -93,4 +93,41 @@ describe 'getting merge request information nested in a project' do
       expect(merge_request_graphql_data['pipelines']['edges'].size).to eq(1)
     end
   end
+
+  context 'when limiting the number of results' do
+    let(:merge_requests_graphql_data) { graphql_data['project']['mergeRequests']['edges'] }
+
+    let!(:merge_requests) do
+      [
+        create(:merge_request, source_project: project, source_branch: 'branch-1'),
+        create(:merge_request, source_project: project, source_branch: 'branch-2'),
+        create(:merge_request, source_project: project, source_branch: 'branch-3')
+      ]
+    end
+
+    let(:fields) do
+      <<~QUERY
+      edges {
+        node {
+          iid,
+          title
+        }
+      }
+      QUERY
+    end
+
+    let(:query) do
+      graphql_query_for(
+        'project',
+        { 'fullPath' => project.full_path },
+        "mergeRequests(first: 2) { #{fields} }"
+      )
+    end
+
+    it 'returns the correct number of results' do
+      post_graphql(query, current_user: current_user)
+
+      expect(merge_requests_graphql_data.size).to eq 2
+    end
+  end
 end