Fix pageInfo for preloaded associations

This corrects the behaviour of the Keyset::Connnection to take
preloaded associations into account.

Adds specs for the keyset connection, and a regression test for
the motivating example. Also ensures that we can paginate.

changelogs/unreleased/ajk-297358-fix-preloaded-pagination.yml

+---
+title: Generate page-info for connections of preloaded associations
+merge_request: 51642
+author:
+type: fixed

lib/gitlab/graphql/pagination/keyset/connection.rb

@@ -67,9 +67,14 @@ module Gitlab
                 # next page
                 true
               elsif first
+                case sliced_nodes
+                when Array
+                  sliced_nodes.size > limit_value
+                else
                   # If we count the number of requested items plus one (`limit_value + 1`),
                   # then if we get `limit_value + 1` then we know there is a next page
                   relation_count(set_limit(sliced_nodes, limit_value + 1)) == limit_value + 1
+                end
               else
                 false
               end
@@ -157,8 +162,8 @@ module Gitlab
 
               list = OrderInfo.build_order_list(items)
 
-              if loaded?(items)
-                @order_list = list.presence || [items.primary_key]
+              if loaded?(items) && !before.present? && !after.present?
+                @order_list = list.presence || [OrderInfo.new(items.primary_key)]
 
                 # already sorted, or trivially sorted
                 next items if list.present? || items.size <= 1
@@ -194,7 +199,7 @@ module Gitlab
             ordering = { 'id' => node[:id].to_s }
 
             order_list.each do |field|
-              field_name = field.attribute_name
+              field_name = field.try(:attribute_name) || field
               field_value = node[field_name]
               ordering[field_name] = if field_value.is_a?(Time)
                                        field_value.strftime('%Y-%m-%d %H:%M:%S.%N %Z')

lib/gitlab/graphql/pagination/keyset/query_builder.rb

@@ -40,7 +40,10 @@ module Gitlab
           #       "issues"."id" > 500
           #
           def conditions
-            attr_values = order_list.map { |field| decoded_cursor[field.attribute_name] }
+            attr_values = order_list.map do |field|
+              name = field.try(:attribute_name) || field
+              decoded_cursor[name]
+            end
 
             if order_list.count == 1 && attr_values.first.nil?
               raise Gitlab::Graphql::Errors::ArgumentError.new('Before/after cursor invalid: `nil` was provided as only sortable value')

spec/lib/gitlab/graphql/pagination/keyset/connection_spec.rb

@@ -21,6 +21,46 @@ RSpec.describe Gitlab::Graphql::Pagination::Keyset::Connection do
     Gitlab::Json.parse(Base64Bp.urlsafe_decode64(cursor))
   end
 
+  context 'regression test for #297358' do
+    let(:projects) { Project.all.preload(:issues) }
+    let(:nodes) { projects.first.issues }
+
+    before do
+      project = create(:project)
+      create_list(:issue, 3, project: project)
+    end
+
+    it 'is loaded' do
+      expect(nodes).to be_loaded
+    end
+
+    it 'does not error when accessing pagination information' do
+      connection.first = 2
+
+      expect(connection).to have_attributes(
+        has_previous_page: false,
+        has_next_page: true
+      )
+    end
+
+    it 'can generate cursors' do
+      connection.send(:ordered_items) # necessary to generate the order-list
+
+      expect(connection.cursor_for(nodes.first)).to be_a(String)
+    end
+
+    it 'can read the next page' do
+      connection.send(:ordered_items) # necessary to generate the order-list
+      ordered = nodes.reorder(id: :desc)
+      next_page = described_class.new(nodes,
+                                      context: context,
+                                      max_page_size: 3,
+                                      after: connection.cursor_for(ordered.second))
+
+      expect(next_page.sliced_nodes).to contain_exactly(ordered.third)
+    end
+  end
+
   it_behaves_like 'a connection with collection methods'
 
   it_behaves_like 'a redactable connection' do

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

@@ -252,4 +252,40 @@ RSpec.describe 'getting merge request information nested in a project' do
       expect(merge_request_graphql_data['mergeStatus']).to eq('checking')
     end
   end
+
+  context 'regression test for #297358' do
+    let(:query) do
+      <<~GQL
+      query($path: ID!) {
+        project(fullPath: $path) {
+          mrs: mergeRequests(first: 1) {
+            nodes {
+              participants { nodes { id } }
+              notes(first: 1) {
+                pageInfo { endCursor hasPreviousPage hasNextPage }
+                nodes { id }
+              }
+            }
+          }
+        }
+      }
+      GQL
+    end
+
+    before do
+      create_list(:note_on_merge_request, 3, project: project, noteable: merge_request)
+    end
+
+    it 'does not error' do
+      post_graphql(query,
+                   current_user: current_user,
+                   variables: { path: project.full_path })
+
+      expect(graphql_data_at(:project, :mrs, :nodes, :notes, :pageInfo)).to contain_exactly a_hash_including(
+        'endCursor' => String,
+        'hasNextPage' => true,
+        'hasPreviousPage' => false
+      )
+    end
+  end
 end