Add Lazy resolution to NullDataloader and remove FlatDataloader

Author: rmosolgo

lib/graphql/dataloader.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-require "graphql/dataloader/flat_dataloader"
 require "graphql/dataloader/null_dataloader"
 require "graphql/dataloader/request"
 require "graphql/dataloader/request_all"

lib/graphql/dataloader/flat_dataloader.rb

@@ -2,17 +2,48 @@
 
 module GraphQL
   class Dataloader
-    # The default implementation of dataloading -- all no-ops.
+    # GraphQL-Ruby uses this when Dataloader isn't enabled.
     #
-    # The Dataloader interface isn't public, but it enables
-    # simple internal code while adding the option to add Dataloader.
+    # It runs execution code inline and gathers lazy objects (eg. Promises)
+    # and resolves them during {#run}.
     class NullDataloader < Dataloader
-      # These are all no-ops because code was
-      # executed synchronously.
+      def initialize(*)
+        @lazies_at_depth = Hash.new { |h,k| h[k] = [] }
+      end
+
+      def freeze
+        @lazies_at_depth.default_proc = nil
+        @lazies_at_depth.freeze
+        super
+      end
+
+      def run(trace_query_lazy: nil)
+        with_trace_query_lazy(trace_query_lazy) do
+          while !@lazies_at_depth.empty?
+            smallest_depth = nil
+            @lazies_at_depth.each_key do |depth_key|
+              smallest_depth ||= depth_key
+              if depth_key < smallest_depth
+                smallest_depth = depth_key
+              end
+            end
+
+            if smallest_depth
+              lazies = @lazies_at_depth.delete(smallest_depth)
+              lazies.each(&:value) # resolve these Lazy instances
+            end
+          end
+        end
+      end
+
+      def run_isolated
+        prev_lazies_at_depth = @lazies_at_depth
+        @lazies_at_depth = @lazies_at_depth.dup.clear
+        yield
+      ensure
+        @lazies_at_depth = prev_lazies_at_depth
+      end
 
-      def initialize(*); end
-      def run(trace_query_lazy: nil); end
-      def run_isolated; yield; end
       def clear_cache; end
       def yield(_source)
         raise GraphQL::Error, "GraphQL::Dataloader is not running -- add `use GraphQL::Dataloader` to your schema to use Dataloader sources."

lib/graphql/dataloader/null_dataloader.rb

@@ -671,7 +671,7 @@ def union_memberships(type = nil)
       # @api private
       # @see GraphQL::Dataloader
       def dataloader_class
-        @dataloader_class || GraphQL::Dataloader::FlatDataloader
+        @dataloader_class || GraphQL::Dataloader::NullDataloader
       end
 
       attr_writer :dataloader_class

lib/graphql/schema.rb

@@ -1451,7 +1451,7 @@ class CanaryDataloader < GraphQL::Dataloader::NullDataloader
       end
       query(query_type)
     end.execute("{ __typename }")
-    assert_instance_of GraphQL::Dataloader::FlatDataloader, res.context.dataloader
+    assert_instance_of GraphQL::Dataloader::NullDataloader, res.context.dataloader
     res = FiberSchema.execute("{ __typename }")
     assert_instance_of GraphQL::Dataloader, res.context.dataloader
     refute res.context.dataloader.nonblocking?

spec/graphql/dataloader_spec.rb

@@ -413,7 +413,7 @@ def execute_multiplex(multiplex:)
         # All of these get `3` as lazy value. They're resolved together,
         # since they aren't _root_ mutation fields.
         "lazyValue" => 3,
-        "i2" => { "value" => 3, "incrementedValue" => 2, "lazyValue" => 3 },
+        "i2" => { "value" => 2, "incrementedValue" => 2, "lazyValue" => 3 },
         "i3" => { "value" => 3, "incrementedValue" => 3, "lazyValue" => 3 },
       },
       "i4" => { "value" => 4, "incrementedValue" => 4, "lazyValue" => 4},