sql_constraints should not be created for AbstractModel

Hi,

The ORM tries to create the SQL contraints on the AbstractModel models.

For an AbstractModel with _sql_contraints:

    class magento_binding(orm.AbstractModel):
        _name = 'magento.binding'
        _description = 'Magento Binding (abstract)'

        _columns = {
            'backend_id': fields.many2one(
                'magento.backend',
                'Magento Backend',
                required=True,
                ondelete='restrict'),
            'magento_id': fields.char('ID on Magento'),
        }

        _sql_constraints = [
            ('magento_uniq', 'unique(backend_id, magento_id)',
             'A record with same ID on Magento already exists.'),
        ]

Error during upgrade of the module:

2013-03-07 08:42:30,796 24916 ERROR openerp_magento7 openerp.sql_db: Programming error: relation "magento_binding" does not exist, in query ALTER TABLE "magento_binding" ADD CONSTRAINT "magento_binding_magento_uniq" unique(backend_id, magento_id

Of course, it can't create the constraint because the table does not exist.