Odoo 12:如何使用datepicker禁用过去的日期？

from odoo import models, fields

class MyModel(models.Model):
    _name = 'my.model'

    date_field = fields.Date(string='Date', required=True, 
                             default=fields.Date.today(), 
                             states={'draft': [('readonly', False)]},
                             help='Select a date')

    @api.onchange('date_field')
    def _onchange_date_field(self):
        today = fields.Date.today()
        if self.date_field and self.date_field < today:
            self.date_field = today