Hi,

the point is that child object is not saved until parent is saved. In order to achieve desired behaviour, you might:

1. Instead of constraints, introduce a check as onchange in validated fields. E.g.

from openerp import exceptions
@api.multi
@api.onchange('field_1','field_2')
def _onchange_field_1(self):
    for object in self:
        if object.field_1 > object.field_2:
             raise exceptions.Warning('NOT GOOD!')

Have a look for a similar example in the 'sale' module, when a partner is changed.

2. Force save of a parent object, when child is saved. It ontradicts Odoo logic! But in the onchange you may add some 'write' function, which launch check of constraints.