только что разобрался.Его ...
asrock = Brand(name='Asrock')
asus = Brand(name='Asus')
mobo = Category(name='Motherboard')
proc = Category(name='Processor')
session.commit()
p1 = Product(name='N98', cost=2100.50, price=2500.50, order_quantity=5, unit='unit',
category=mobo, brand=asrock)
p2 = Product(name='M-N98', cost=2300.50, price=2500.50, order_quantity=5, unit='unit',
category=mobo, brand=asus)
session.commit()