class Child(models.Model):

    _name = 'model.child' 

class A(models.Model):

    _name = 'model.a' 

    _inherits = {'model.child':'child_id'}

    child_id = fields.Many2one('model.child', required=True, ondelete='cascade')

@api.multi

def unlink(self):

for model_a in self:

model_a.child_id.unlink()

I always get an Access Error. The requested operation cannot be completed due to security restrictions. Please contact your system administrator.
(Document type: Message, Operation: unlink)

With odoo 11 it was not a problem, but with odoo 12 I cannot delete the delegated child. How can it be solved?