Add subscription support to @apollo/subgraph

.changeset/spotty-hornets-provide.md

@@ -0,0 +1,6 @@
+---
+"@apollo/subgraph": patch
+---
+
+Correctly attach provided subscription resolvers to the schema object
+

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

@@ -1005,6 +1005,38 @@ describe('buildSubgraphSchema', () => {
     expect(() => buildSubgraphSchema(doc)).not.toThrow();
   });
 
+  it('correctly attaches the provided subscribe function to the schema object', () => {
+    async function* subscribeFn () {
+      for await (const word of ['Hello', 'Bonjour', 'Ciao']) {
+        yield word;
+      }
+    }
+    const schema = buildSubgraphSchema([
+      {
+        typeDefs: gql`
+          type Query {
+            hello: String!
+          }
+
+          type Subscription {
+            hello: String!
+          }
+        `,
+        resolvers: {
+          Subscription: {
+            hello: {
+              subscribe: subscribeFn,
+            },
+          },
+        },
+      },
+    ]);
+
+    expect(schema.getSubscriptionType()?.getFields()['hello'].subscribe).toBe(
+      subscribeFn,
+    );
+  });
+
   // Those tests ensures that we expand older federation specification to their proper definitions,
   // so they explicitely link to older spec and should not be changed.
   describe('federation specification backward compatibility', () => {

subgraph-js/src/schema-helper/buildSchemaFromSDL.ts

@@ -195,6 +195,7 @@ export function addResolversToSchema(
         field.resolve = fieldConfig;
       } else {
         field.resolve = fieldConfig.resolve;
+        field.subscribe = fieldConfig.subscribe;
       }
     }
   }

subgraph-js/src/schema-helper/resolverMap.ts

@@ -13,7 +13,8 @@ export interface GraphQLResolverMap<TContext = {}> {
           | GraphQLFieldResolver<any, TContext>
           | {
               requires?: string;
-              resolve: GraphQLFieldResolver<any, TContext>;
+              resolve?: GraphQLFieldResolver<any, TContext>;
+              subscribe?: GraphQLFieldResolver<any, TContext>;
             };
       }
     | GraphQLScalarType