Let's assume we have a Dogs application, then our model will be a Dog class.

DatabaseReference reference = FirebaseDatabase.getInstance().getReference().child("dogs");

This is how to send a Dog to the database, a new unique dog and set the dog with the key.

String key = reference.push().getKey();
Dog dog = new Dog("Spike");
dog.setKey(key);
reference.child(key).setValue(dog);

The reference.child(key).setValue(dog); is equivalent of reference.push().setValue(dog); And add the benefit of getting the key inside the Dog object.