Store the document only once when several field keys are supplied

lib/absinthe/phase/subscription/subscribe_self.ex

@@ -25,8 +25,7 @@ defmodule Absinthe.Phase.Subscription.SubscribeSelf do
       field_keys = get_field_keys(field, config)
       subscription_id = get_subscription_id(config, blueprint, options)
 
-      for field_key <- field_keys,
-          do: Absinthe.Subscription.subscribe(pubsub, field_key, subscription_id, blueprint)
+      Absinthe.Subscription.subscribe(pubsub, field_keys, subscription_id, blueprint)
 
       {:replace, blueprint,
        [

lib/absinthe/subscription.ex

@@ -140,25 +140,31 @@ defmodule Absinthe.Subscription do
   defp fetch_fields(_, _), do: []
 
   @doc false
-  def subscribe(pubsub, field_key, doc_id, doc) do
+  def subscribe(pubsub, field_keys, doc_id, doc) do
+    field_keys = List.wrap(field_keys)
+
     registry = pubsub |> registry_name
 
     doc_value = %{
       initial_phases: PipelineSerializer.pack(doc.initial_phases),
       source: doc.source
     }
 
-    pdict_add_field(doc_id, field_key)
-    {:ok, _} = Registry.register(registry, field_key, doc_id)
+    pdict_add_fields(doc_id, field_keys)
+
+    for field_key <- field_keys do
+      {:ok, _} = Registry.register(registry, field_key, doc_id)
+    end
+
     {:ok, _} = Registry.register(registry, doc_id, doc_value)
   end
 
   defp pdict_fields(doc_id) do
     Process.get({__MODULE__, doc_id}, [])
   end
 
-  defp pdict_add_field(doc_id, field) do
-    Process.put({__MODULE__, doc_id}, [field | pdict_fields(doc_id)])
+  defp pdict_add_fields(doc_id, field_keys) do
+    Process.put({__MODULE__, doc_id}, field_keys ++ pdict_fields(doc_id))
   end
 
   defp pdict_delete_fields(doc_id) do