Add __resolveType to _Entity union (#1773)

subgraph-js/CHANGELOG.md

@@ -6,7 +6,7 @@ This CHANGELOG pertains only to Apollo Federation packages in the `0.x` range. T
 
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
-- _Nothing yet! Stay tuned._
+- Add __resolveType to _Entity union [PR #1778](https://github.com/apollographql/federation/pull/1778)
 
 ## v0.4.0
 - Support for Node 17 [PR #1648](https://github.com/apollographql/federation/pull/1648).

subgraph-js/src/__tests__/buildSubgraphSchema.test.ts

@@ -1,5 +1,11 @@
 import gql from 'graphql-tag';
-import { Kind, graphql, DocumentNode, execute } from 'graphql';
+import {
+  Kind,
+  graphql,
+  DocumentNode,
+  execute,
+  GraphQLUnionType,
+} from 'graphql';
 import { buildSubgraphSchema } from '../buildSubgraphSchema';
 import { typeSerializer } from 'apollo-federation-integration-testsuite';
 
@@ -625,6 +631,15 @@ type Product {
       `union _Entity = Product`,
     );
   });
+
+  it('defines the `resolveType` resolver on the `_Entity` union', async () => {
+    const schema = buildSubgraphSchema({ typeDefs });
+
+    expect(
+      (schema.getType('_Entity') as GraphQLUnionType).resolveType,
+    ).toBeDefined();
+  });
+
   it('allows legacy schema module interface as a simple array of documents', async () => {
     const schema = buildSubgraphSchema({ typeDefs });
     expect(schema.getType('Product')).toMatchInlineSnapshot(`

subgraph-js/src/buildSubgraphSchema.ts

@@ -122,6 +122,9 @@ export function buildSubgraphSchema(
       return new GraphQLUnionType({
         ...EntityType.toConfig(),
         types: entityTypes.filter(isObjectType),
+        resolveType(parent: { __typename: string }) {
+          return parent.__typename;
+        },
       });
     }
     return undefined;