mutate
Override this method to provide your own implementation of data mutator.
This method can be invoked more than once in case of data collision, i.e. if data on backend is modified while the mutate operation is executing.
This method is invoked on a background thread.
Return
New contents of the entity as a serialized JSON object that will replace the old contents or null to abort the edit entirely.
Parameters
current
Current contents of the entity to modify as a serialized JSON object