Support recursive schema references by allowing quoted fields

As reported in Issue graphql-elixir#38, Schemas weren't able to recursively refer to
one another. By quoting the offending references and only evaling them
during Execution, we can resolve everything on-demand.

lib/graphql/execution/executor.ex

@@ -156,9 +156,15 @@ defmodule GraphQL.Execution.Executor do
     end
   end
 
+  defp maybe_unwrap(item) when is_tuple(item) do
+    {result,_} = Code.eval_quoted(item)
+    result
+  end
+  defp maybe_unwrap(item), do: item
+
   defp field_definition(_schema, parent_type, field_name) do
     # TODO deal with introspection
-    parent_type.fields[field_name]
+    maybe_unwrap(parent_type.fields)[field_name]
   end
 
   defp argument_values(arg_defs, arg_asts, variable_values) do

test/graphql/execution/executor_test.exs

@@ -8,6 +8,31 @@ defmodule GraphQL.Execution.Executor.ExecutorTest do
   alias GraphQL.Execution.Executor
 
   defmodule TestSchema do
+    def recursive_schema do
+      %GraphQL.Schema{
+        query: %GraphQL.ObjectType{
+          name: "Recursive1",
+          fields: quote do %{
+            id:   %{type: "Integer", resolve: 1},
+            name: %{type: "String", resolve: "Mark"},
+            b: %{type: TestSchema.recursive_schema.query },
+            c: %{type: TestSchema.recursive_schema_2 }
+          } end
+        }
+      }
+    end
+
+    def recursive_schema_2 do
+      %GraphQL.ObjectType{
+        name: "Recursive2",
+        fields: quote do %{
+          id:   %{type: "Integer", resolve: 2},
+          name: %{type: "String", resolve: "Kate"},
+          b: %{type: TestSchema.recursive_schema.query }
+        } end
+      }
+    end
+
     def schema do
       %GraphQL.Schema{
         query: %GraphQL.ObjectType{
@@ -73,6 +98,11 @@ defmodule GraphQL.Execution.Executor.ExecutorTest do
     assert_execute {"{ a }", TestSchema.schema}, %{}
   end
 
+  test "Quoted fields are available" do
+    assert_execute({"{id, b { name, c{ id, name, b { name }}}}", TestSchema.recursive_schema},
+        %{id: 1, b: %{name: "Mark", c: %{id: 2, name: "Kate", b: %{name: "Mark"}}}})
+  end
+
   test "simple selection set" do
     schema = %GraphQL.Schema{
       query: %GraphQL.ObjectType{