Select extra node in pagination relations for free hasNextPage query

lib/graphql/pagination/relation_connection.rb

@@ -221,7 +221,22 @@ def limited_nodes
       # returns an array of nodes
       def load_nodes
         # Return an array so we can consistently use `.index(node)` on it
-        @nodes ||= limited_nodes.to_a
+        @nodes ||= begin
+          original_node_limit = relation_limit(limited_nodes)
+          if original_node_limit
+            overshot_nodes = set_limit(limited_nodes, original_node_limit + 1).to_a
+            if overshot_nodes.size > original_node_limit
+              @has_next_page = true
+              overshot_nodes[0...-1]
+            else
+              # we didn't overshoot and there are no next pages
+              @has_next_page = false
+              overshot_nodes
+            end
+          else
+            limited_nodes.to_a
+          end
+        end
       end
     end
   end

spec/graphql/pagination/active_record_relation_connection_spec.rb

@@ -159,7 +159,10 @@ def total_count
       assert_equal ["Item"] * 10, results["data"]["items"]["nodes"].map { |i| i["__typename"] }
       assert_equal 1, log.split("\n").size, "It runs only one query when less than total count is requested"
       assert_equal 0, log.scan("COUNT(").size, "It runs no count query"
+    end
 
+    it "runs one query for both `nodes` and `hasNextPage` if nodes happen to be selected first" do
+      results = nil
       log = with_active_record_log do
         results = schema.execute("{
           items(first: 3) {
@@ -172,10 +175,28 @@ def total_count
           }
         }")
       end
-      # This currently runs one query to load the nodes, then another one to count _just beyond_ the nodes.
-      # A better implementation would load `first + 1` nodes and use that to set `has_next_page`.
       assert_equal ["Item", "Item", "Item"], results["data"]["items"]["nodes"].map { |i| i["__typename"] }
-      assert_equal 2, log.split("\n").size, "It runs two queries -- TODO this could be better"
+      assert_equal 1, log.split("\n").size, "It runs only one query"
+      assert_equal 0, log.scan("COUNT(").size, "It runs no count query"
+    end
+
+    it "runs two queries for `nodes` and `hasNextPage` if hasNextPage is selected first" do
+      results = nil
+      log = with_active_record_log do
+        results = schema.execute("{
+          items(first: 3) {
+            pageInfo {
+              hasNextPage
+            }
+            nodes {
+              __typename
+            }
+          }
+        }")
+      end
+      assert_equal ["Item", "Item", "Item"], results["data"]["items"]["nodes"].map { |i| i["__typename"] }
+      assert_equal 2, log.split("\n").size, "It runs two queries"
+      assert_equal 1, log.scan("COUNT(").size, "It runs one count query"
     end
 
     describe "already-loaded ActiveRecord relations" do