@api.constrains('value')

.....

....
raise ValidationError(_("Your record is too small: %s") % record.value)

try this one. It's correct form

Try this,

from openerp import models, fields, api

from openerp.exceptions import ValidationError

class HelloWorld(models.Model):

     _name = 'helloworld.test'

    name = fields.Char("Name", required=True, size=20)

    value = fields.Integer("Value", required=True)

    @api.onchange('value')

    def _onchange_value(self)

        if self.value < 20:

            raise ValidationError("Your record is too small: %s" % self.value)

@api.constrains('value')

def _check_value(self)

        if self.value < 20:

            raise ValidationError("Your record is too small: %s" % self.value)

There was a typo... problem solved