Allow mutations to return custom classes

src/mutation/__tests__/mutation-test.js

@@ -195,9 +195,43 @@ describe('mutationWithClientMutationId()', () => {
       }
     `;
 
+    expect(graphqlSync({ schema, source })).to.deep.equal({
+      data: { someMutation: null },
+    });
+  });
+
+  it('supports mutations returning custom classes', () => {
+    class SomeClass {
+      getSomeGeneratedData() {
+        return 1;
+      }
+    }
+
+    const someMutation = mutationWithClientMutationId({
+      name: 'SomeMutation',
+      inputFields: {},
+      outputFields: {
+        result: {
+          type: GraphQLInt,
+          resolve: (obj) => obj.getSomeGeneratedData(),
+        },
+      },
+      mutateAndGetPayload: () => new SomeClass(),
+    });
+    const schema = wrapInSchema({ someMutation });
+
+    const source = `
+      mutation {
+        someMutation(input: {clientMutationId: "abc"}) {
+          result
+          clientMutationId
+        }
+      }
+    `;
+
     expect(graphqlSync({ schema, source })).to.deep.equal({
       data: {
-        someMutation: { result: null, clientMutationId: 'abc' },
+        someMutation: { result: 1, clientMutationId: 'abc' },
       },
     });
   });

src/mutation/mutation.js

@@ -91,10 +91,18 @@ export function mutationWithClientMutationId(
       const { clientMutationId } = input;
       const payload = mutateAndGetPayload(input, context, info);
       if (isPromise(payload)) {
-        return payload.then((data) => ({ ...data, clientMutationId }));
+        return payload.then(injectClientMutationId);
       }
+      return injectClientMutationId(payload);
 
-      return { ...payload, clientMutationId };
+      function injectClientMutationId(data: mixed) {
+        if (typeof data === 'object' && data !== null) {
+          // $FlowFixMe[cannot-write] It's bad idea to mutate data but we need to pass clientMutationId somehow. Maybe in future we figure out better solution satisfying all our test cases.
+          data.clientMutationId = clientMutationId;
+        }
+
+        return data;
+      }
     },
   };
 }