raise TypeError("Mixing apples and oranges: %s & %s" % (self, other))

TypeError: Mixing apples and oranges: purchase.order(<odoo.models.NewId object at 0x7f726469aac8>,) & sub.contract(<odoo.models.NewId object at 0x7f726469aac8>,)

I have inherit purchase.order as test.order. and the view is same as coppied from purchase order form.

The above error happened when i select the order_line field (any field value).

My Code is:

class TestOrder(models.Model):
    _name = "test.order"
    _inherit = 'purchase.order'
    
    analytic_account_id = fields.Many2one('account.analytic.account', string="Project")
    category = fields.Many2one('test.order.category', string="Category")
    code = fields.Many2one('test.order.category', string="Cost Code")

XML Code is same as purchase.order form( id="purchase_order_form")