I wanted to create a membership child table for res.partner. I wanted to use delegation (_inherits) so that child fields can also be accessed from the parent res.partner table :

class MyMembership(models.Model):
	_name = "mymembership"        
	memberid = fields.Char(string="Membership ID")    
	member_expiry = fields.Date(string="Expiry Date")​

class MembershipCustomer(models.Model):
	_inherit = "res.partner"
	_inherits = {
		'mymembership': 'partner_member_id',
	}
	partner_member_id = fields.Many2one('mymembership', ondelete='cascade')

The requested operation cannot be completed due to security restrictions. Please contact your system administrator.
(Document type: res.partner, Operation: read)
​

What did i miss ?