I'm not sure I understand. That is very common. An example from the book
Overview:
db.define_table('image',
Field <http://web2py.com/book/default/docstring/Field>('title'),
Field <http://web2py.com/book/default/docstring/Field>('file', 'upload'))
db.define_table('comment',
Field <http://web2py.com/book/default/docstring/Field>('image_id', db.image),
Field <http://web2py.com/book/default/docstring/Field>('author'),
Field <http://web2py.com/book/default/docstring/Field>('email'),
Field <http://web2py.com/book/default/docstring/Field>('body', 'text'))
