#mydev.py
from trytond.model import ModelView, ModelSQL, fields
class Theparent(ModelSQL, ModelView):
    'TheParent'
    _name = 'mydev.theparent'
    _description = __doc__
    name = fields.Char('Parent Name')
    desc = fields.Char('Parent Desc')

Theparent()

class Thechild(ModelSQL, ModelView):
    'TheChild'
    _name = 'mydev.thechild'
    _description = __doc__
    theparent = fields.Many2One('mydev.theparent', 'Theparent',
            required=True,
            ondelete='CASCADE')
    name = fields.Char('Child Name')
    desc = fields.Char('Child Desc')

Thechild()