Adds support for providing the type of an object when the type is a union

Author: jonbretman

src/execution/execute.js

@@ -1034,9 +1034,17 @@ function completeAbstractValue(
   path: ResponsePath,
   result: mixed
 ): mixed {
-  const runtimeType = returnType.resolveType ?
-    returnType.resolveType(result, exeContext.contextValue, info) :
-    defaultResolveTypeFn(result, exeContext.contextValue, info, returnType);
+  let runtimeType;
+
+  if (result && result.__type) {
+    runtimeType = result.__type;
+  } else if (returnType.resolveType) {
+    runtimeType = returnType.resolveType(result, exeContext.contextValue, info);
+  } else {
+    runtimeType = defaultResolveTypeFn(
+      result, exeContext.contextValue, info, returnType
+    );
+  }
 
   const promise = getPromise(runtimeType);
   if (promise) {

src/utilities/__tests__/buildASTSchema-test.js

@@ -394,6 +394,67 @@ describe('Schema Builder', () => {
     expect(output).to.equal(body);
   });
 
+  it('Specifying which type to use for union types using __type', async () => {
+    const schema = buildSchema(`
+      schema {
+        query: Root
+      }
+
+      type Root {
+        fruits: [Fruit]
+      }
+
+      union Fruit = Apple | Banana
+
+      type Apple {
+        color: String
+      }
+
+      type Banana {
+        length: Int
+      }
+    `);
+
+    const query = `
+      {
+        fruits {
+          ... on Apple {
+            color
+          }
+          ... on Banana {
+            length
+          }
+        }
+      }
+    `;
+
+    const root = {
+      fruits: [
+        {
+          color: 'green',
+          __type: 'Apple',
+        },
+        {
+          length: 5,
+          __type: 'Banana',
+        }
+      ]
+    };
+
+    expect(await graphql(schema, query, root)).to.deep.equal({
+      data: {
+        fruits: [
+          {
+            color: 'green',
+          },
+          {
+            length: 5,
+          }
+        ]
+      }
+    });
+  });
+
   it('Custom Scalar', () => {
     const body = dedent`
       schema {