If you want to catch SQL constraints violations in OpenERP/Odoo controllers this tutorial will help you,

So, You can add an SQL constraint to Odoo model without facing any difficulty, to do this we have to use the _sql_constraints list. Odoo Forms handle those constraints automatically in pgadmin.

let's consider the below example:

class ShivaController(http.Controller):
    @http.route('/Shiva/create/', auth='public', website=True)
    def create(self, foo, bar):
        try:
            http.request.env['my.Shiva'].create({
                'foo': foo,
                'bar': bar,
            })
            return http.request.render('my.thank_you_page')
        except IntegrityError:
           
            http.request._cr.rollback()
            return http.request.render('my.error_page')

Note- In SQL constraints current transaction is aborted, commands ignored until end of transaction block.

And after changing SQL constraints we have to inherit SQL Constraints from odoo 7 moduels to Odoo 8, see the example shown below:

class Findnerd(models.Model):
    _inherit = 'my.Findnerd '

    # ...

    _sql_constraints = [
        ('name_uniq', 'unique(name)', 'Two foos with the same name? Impossible!')
    ]

    def _auto_init(self, cr, context=None):
        self._sql_constraints = self._sql_constraints.reverse()
        super(Findnerd, self)._auto_init(cr, context)