Ensure root objects are marked as referenced correctly

Followup to absinthe-graphql#1176

Since every schema will have a declared or inferred declaration, this
can be used to mark root objects as referenced.

The MarkReferenced used to mark every :query/:subscription/:mutation object
as referenced, even when it was used in the declaration.

This fix ensures that root objects that are not used in the declaration
are not marked as referenced and thus not rendered in the SDL.

Author: maartenvanvliet

lib/absinthe/phase/schema/mark_referenced.ex

@@ -9,13 +9,16 @@ defmodule Absinthe.Phase.Schema.MarkReferenced do
     %{schema_definitions: [schema]} = blueprint
 
     schema =
-      Map.update!(schema, :type_definitions, &mark_referenced(&1, schema.directive_definitions))
+      Map.update!(
+        schema,
+        :type_definitions,
+        &mark_referenced(&1, schema.directive_definitions, schema.schema_declaration)
+      )
 
     {:ok, %{blueprint | schema_definitions: [schema]}}
   end
 
-  @roots [:query, :mutation, :subscription]
-  defp mark_referenced(type_defs, directive_defs) do
+  defp mark_referenced(type_defs, directive_defs, schema_declaration) do
     types_by_ref =
       Enum.reduce(type_defs, %{}, fn type_def, acc ->
         acc
@@ -24,8 +27,8 @@ defmodule Absinthe.Phase.Schema.MarkReferenced do
       end)
 
     referenced_type_ids =
-      @roots
-      |> Enum.map(&Map.get(types_by_ref, &1))
+      schema_declaration.field_definitions
+      |> Enum.map(&Map.get(types_by_ref, &1.identifier))
       |> Enum.reject(&is_nil/1)
       |> Enum.concat(directive_defs)
       |> Enum.reduce(MapSet.new(), &referenced_types(&1, types_by_ref, &2))

test/absinthe/schema_test.exs

@@ -328,10 +328,6 @@ defmodule Absinthe.SchemaTest do
                name: String
              }
 
-             type MyRootMutation {
-               name: String
-             }
-
              type RootQueryType {
                name(familyName: Boolean): String
              }