Add __resolveType to _Entity union (#1773)

Author: michael-watson

subgraph-js/CHANGELOG.md

@@ -2,6 +2,8 @@
 
 This CHANGELOG pertains only to Apollo Federation packages in the 2.x range. The Federation v0.x equivalent for this package can be found [here](https://github.com/apollographql/federation/blob/version-0.x/subgraph-js/CHANGELOG.md) on the `version-0.x` branch of this repo.
 
+- Add __resolveType to _Entity union [PR #1773](https://github.com/apollographql/federation/pull/1773)
+
 ## v2.0.1
 
 - Released in sync with other federation packages but no changes to this package.

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

@@ -1,5 +1,13 @@
 import gql from 'graphql-tag';
-import { Kind, graphql, DocumentNode, execute, type DefinitionNode, OperationTypeNode } from 'graphql';
+import {
+  Kind,
+  graphql,
+  DocumentNode,
+  execute,
+  type DefinitionNode,
+  OperationTypeNode,
+  GraphQLUnionType,
+} from 'graphql';
 import { buildSubgraphSchema } from '../buildSubgraphSchema';
 import { typeSerializer } from 'apollo-federation-integration-testsuite';
 import { errorCauses } from '@apollo/federation-internals';
@@ -1046,6 +1054,15 @@ 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

@@ -76,6 +76,11 @@ export function buildSubgraphSchema(
     addResolversToSchema(schema, {
      [queryRootName] : {
         _entities: (_source, { representations }, context, info) => entitiesResolver({ representations, context, info }),
+      },
+      _Entity: {
+        __resolveType(parent: { __typename: string }) {
+          return parent.__typename;
+        },
       }
     });
   }