I have created a new module and I inherited like this

class Course(models.Model):

    _name = 'openacademy.course'

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

    description = fields.Text()

    responsible_id = fields.Many2one('res.users',ondelete='set null', string="Responsible", index=True)

    purchase_id = fields.Many2one('purchase.order', string="Price")

------------------After upgration when I click the dropdown list in the Price its shows error like this

ProgrammingError: relation "_unknown" does not exist
LINE 1: SELECT "_unknown".id FROM "_unknown" ORDER BY "_unknown"."id...
                                  ^