Rational
When it create a record this return a instance object, this process is fine (it could be slow), but sometimes we need only that returns the ids of new records not all instance object for example for to create hundred/thousands of records in batch process, maybe would be a faster transaction and less works in server.
Proposal
Add a new method for to create or add to create method an attribute for return only ids.
Implementation
Add new method ModelStorage.create_with_ids(values)
or
add new attribute to ModelStorage.create(values, return_ids=True/False)