Now we know whether the primary key is defined or not
and we do not allow to make update/remove if the key is not defined.
And when doing insert/update we can put NULL if child models don't have
the primary key set (fields with has_foreign_key set to true).
Now in after_select() we should also set has_primary_key_set flag
or just call get_last_sequence_for_primary_key instead of get_last_sequence.
fixed: added prefix +00 when serializing PT::Date to PostgreSQL (time zone)
(for a column with a time zone there was a wrong value saved)