Hi,

Check whether you have defined any many2one field or many2many field inside the same model, if yes, you might not have set the comodel for the filed.

partner_id = fields.Many2one('res.partner', string='Customer')

like this you have to define the many2one field, but you might not have specified the comodel, you might added the field like this,

partner_id = fields.Many2one(string='Customer')

Thanks