If the database schema is not compatible with this bean and
RedBean runs in frozen mode it will throw an exception.
This function returns the primary key ID of the inserted
bean.
The return value is an integer if possible. If it is not possible to
represent the value as an integer a string will be returned. We use
explicit casts instead of functions to preserve performance
(0.13 vs 0.28 for 10000 iterations on Core i3).