Support for union types when using buildSchema (#947)

Closes #947

Squashed commit of the following:

commit 8d19d2634d728abfe61a50070259c588096343e2
Author: Lee Byron <[email protected]>
Date:   Fri Dec 1 13:45:24 2017 -0800

    Clean up existing tests and improve error messages

commit 95018b1a8484dea64f29faf48f111a3906df348e
Author: Lee Byron <[email protected]>
Date:   Fri Dec 1 13:29:45 2017 -0800

    Move resolveType __typename checking into defaultResolveType

commit 1af7646053dd426d9fe43e10e6ecd3ab7c1905a8
Author: jonbretman <[email protected]>
Date:   Thu Jul 13 11:16:40 2017 +0100

    Adds support for resolving union/interface types when using a generated schema

Author: leebyron

src/execution/execute.js

@@ -1092,7 +1092,9 @@ function ensureValidRuntimeType(
     throw new GraphQLError(
       `Abstract type ${returnType.name} must resolve to an Object type at ` +
       `runtime for field ${info.parentType.name}.${info.fieldName} with ` +
-      `value "${String(result)}", received "${String(runtimeType)}".`,
+      `value "${String(result)}", received "${String(runtimeType)}". ` +
+      `Either the ${returnType.name} type should provide a "resolveType" ` +
+      'function or each possible types should provide an "isTypeOf" function.',
       fieldNodes
     );
   }
@@ -1197,15 +1199,30 @@ function collectAndExecuteSubfields(
 
 /**
  * If a resolveType function is not given, then a default resolve behavior is
- * used which tests each possible type for the abstract type by calling
+ * used which attempts two strategies:
+ *
+ * First, See if the provided value has a `__typename` field defined, if so, use
+ * that value as name of the resolved type.
+ *
+ * Otherwise, test each possible type for the abstract type by calling
  * isTypeOf for the object being coerced, returning the first type that matches.
  */
 function defaultResolveTypeFn(
   value: mixed,
   context: mixed,
   info: GraphQLResolveInfo,
   abstractType: GraphQLAbstractType
-): ?GraphQLObjectType | Promise<?GraphQLObjectType> {
+): ?GraphQLObjectType | string | Promise<?GraphQLObjectType | string> {
+  // First, look for `__typename`.
+  if (
+    value !== null &&
+    typeof value === 'object' &&
+    typeof value.__typename === 'string'
+  ) {
+    return value.__typename;
+  }
+
+  // Otherwise, test each possible type.
   const possibleTypes = info.schema.getPossibleTypes(abstractType);
   const promisedIsTypeOfResults = [];
 

src/type/__tests__/definition-test.js

@@ -89,10 +89,7 @@ const BlogSubscription = new GraphQLObjectType({
   }
 });
 
-const ObjectType = new GraphQLObjectType({
-  name: 'Object',
-  isTypeOf: () => true
-});
+const ObjectType = new GraphQLObjectType({ name: 'Object' });
 const InterfaceType = new GraphQLInterfaceType({ name: 'Interface' });
 const UnionType =
   new GraphQLUnionType({ name: 'Union', types: [ ObjectType ] });
@@ -281,7 +278,6 @@ describe('Type System: Example', () => {
         f: { type: GraphQLInt }
       },
       interfaces: [ SomeInterface ],
-      isTypeOf: () => true
     });
 
     const schema = new GraphQLSchema({
@@ -311,7 +307,6 @@ describe('Type System: Example', () => {
         f: { type: GraphQLInt }
       },
       interfaces: () => [ SomeInterface ],
-      isTypeOf: () => true
     });
 
     const schema = new GraphQLSchema({

src/type/__tests__/schema-test.js

@@ -18,9 +18,6 @@ import { expect } from 'chai';
 const InterfaceType = new GraphQLInterfaceType({
   name: 'Interface',
   fields: { fieldName: { type: GraphQLString } },
-  resolveType() {
-    return ImplementingType;
-  }
 });
 
 const ImplementingType = new GraphQLObjectType({