References

When you add a user or a collection object to an activity, Stream stores the unique reference and replaces it at read time. In some complex cases, you need to be able to generate a reference to an existing object and embed that inside of an activity.

// First create a collection entry with upsert api
client.collections().upsert("food", new CollectionData().set("name", "Cheese Burger")).join();

// Then create a user
client.user("john-doe").create(new Data()
    .set("name", "John Doe")
    .set("occupation", "Software Engineer")
    .set("gender", "male")).join();

// Since we know their IDs we can create references to both without reading from APIs
String cheeseBurgerRef = createCollectionReference("food", "cheese-burger");
String johnDoeRef = createUserReference("john-doe");

client.flatFeed("user", "john").addActivity(Activity.builder()
    .actor(johnDoeRef)
    .verb("eat")
    .object(cheeseBurgerRef)
    .build()).join();

Info:

If you are using the APIs on web / mobile (see auth section) you must set activity.actor to the reference of the current user or otherwise you will get a permission error (see examples above).