Write-once read-only field

We need to implement a write-once (on object create) read-only field with SQLAlchemy/Elixir.

A quick-and-dirty solution:

class User(Entity):
    # some fields ...    
    _created    = Field(DateTime, default=datetime.utcnow)
    @property
    def created(self):
        return self._created

Is there a better approach (e.g., using before-insert triggers?)