I use this;

class A (models.Model)

_name =  'A'

.....

class B (models.Model)

_name = 'B'

_inherits = { 'A':'a_id'}

a_id = fields.Many2one('a',required=True,ondelete='cascade')

......

@api.multi

def unlink(self):

self.a_id.unlink()

return super(B,self).unlink()

but get this Error "Record does not exist or has been deleted."

How to delete A data from B tables