Adds support for resolving a union type when using a generated schema

Author: jonbretman

src/utilities/__tests__/buildASTSchema-test.js

@@ -394,6 +394,67 @@ describe('Schema Builder', () => {
     expect(output).to.equal(body);
   });
 
+  it('Specifying union type using __typename', 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',
+          __typename: 'Apple',
+        },
+        {
+          length: 5,
+          __typename: 'Banana',
+        }
+      ]
+    };
+
+    expect(await graphql(schema, query, root)).to.deep.equal({
+      data: {
+        fruits: [
+          {
+            color: 'green',
+          },
+          {
+            length: 5,
+          }
+        ]
+      }
+    });
+  });
+
   it('Custom Scalar', () => {
     const body = dedent`
       schema {

src/utilities/buildASTSchema.js

@@ -437,7 +437,7 @@ export function buildASTSchema(ast: DocumentNode): GraphQLSchema {
       name: def.name.value,
       description: getDescription(def),
       types: def.types.map(t => produceObjectType(t)),
-      resolveType: cannotExecuteSchema,
+      resolveType: resolveUnionType,
       astNode: def,
     });
   }
@@ -530,6 +530,16 @@ function leadingSpaces(str) {
   return i;
 }
 
+function resolveUnionType(result) {
+  if (result && result.__typename) {
+    return result.__typename;
+  }
+  throw new Error(
+    'To resolve union types using a generated schema the result must have ' +
+    'a __typename property corresponding to the exact type used.'
+  );
+}
+
 function cannotExecuteSchema() {
   throw new Error(
     'Generated Schema cannot use Interface or Union types for execution.'