true if the model has been deleted, and false otherwise.
In general, it's best to avoid keeping a reference to an object past the
current event loop, since it may be deleted and trying to access any data
of a deleted object (other than its ID) will throw. But if you keep a
reference, you can use isDeleted to check that it's safe to access the
model's data.
toString
function() => string
A string representation of the model for use in debugging.