openERP function for validate date range

I have two fields in my module,(start_date and end_date). I want to validate the date range as end_date must be greater than start_date and display error message like "End Date Should be greater than Start Date" This is mu cord.

from openerp.osv import osv, fields


class op_batch(osv.Model):

    _name = 'op.batch'
    _columns = {
        'name': fields.char(size=25, string='Name', required=True),
        'code': fields.char(size=15, string='Code', required=True),
        'start_date': fields.date(size=15, string='Start Date', required=True),
        'end_date': fields.date(size=15, string='End Date', required=True, onchange="validate_date_range"),
        'state': fields.selection(
        [('planned', 'Planned'), ('running', 'Running'), ('cancel', 'Cancel'), ('finished', 'finished')],
        string='State'),
        'course_id': fields.many2one('op.course', string='Course', ondelete='restrict', required=True),
}

    def validate_date_range(self, cr, uid, vals, context=None):
        en_date = date.start_date.value
        st_date = date.end_date.value
        if en_date < st_date:
            raise Warning(_("End Date Should be greater than Start Date"))
    return True

    _sql_constraints = [('code', 'UNIQUE (code)', 'The CODE of the Batch must be unique!')]

    _defaults = {
    'state': 'planned',
    }

how should I do that? Please Help me to do this...

To enforce data integrity, odoo support two types of constraints: SQL and Python.

SQL constraints are added to the table definition in the database and implemented by PostgreSQL. They are defined using the class attribute _sql_constraints. It is a list of tuples with the constraint identifier name, the SQL for the constraint, and the error message to use.

Python Constraint

In v7 api,

_constraint is a collection of list of tuples.

Tuple contains three parameters,

1. Method Name (where your actual logic coded)
2. The Validation Message (Message that you want to show to user)
3. List of Fields (fields to which you want to apply the constraint)

_constraint will raise the validation message if the condition returns False on create/update record.

Just add constraints for that,

def _check_date(self, cr, uid, vals, context=None):
    for obj in self.browse(cr, uid, ids):
        start_date = obj.start_date
        end_date = obj.end_date

        if start_date and end_date:
            DATETIME_FORMAT = "%Y-%m-%d"  ## Set your date format here
            from_dt = datetime.datetime.strptime(start_date, DATETIME_FORMAT)
            to_dt = datetime.datetime.strptime(end_date, DATETIME_FORMAT)

            if to_dt < from_dt:
                return False
    return True

_constraints = [
        (_check_date, 'Your Message!', ['start_date','end_date']),
    ]

In v8 api,

@api.constrains

This decorator will ensure that decorated function will be called on create, write, unlink operation. If a constraint is met the function should raise a openerp.exceptions.Warning with appropriate message.

@api.multi
@api.constrains('start_date','end_date')
def _check_date(self):
    for obj in self:
        start_date = obj.start_date
        end_date = obj.end_date

        if start_date and end_date:
            DATETIME_FORMAT = "%Y-%m-%d"  ## Set your date format here
            from_dt = datetime.datetime.strptime(start_date, DATETIME_FORMAT)
            to_dt = datetime.datetime.strptime(end_date, DATETIME_FORMAT)

            if to_dt < from_dt:
                #raise your exception

If you want to change the existing constraints, it can be done by using inheritance see here