django - Signal for M2M changed on a through field -

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before switching m2m field through field had signal call methods on instance update of values. switched 'through' type m2m , stopped functioning. have work around calling functions in serializer, kind of nasty. how fix it?

signal

@receiver(m2m_changed, sender=order.items.through) def update_order_when_items_changed(sender, instance, **kwargs):     instance.set_weight()     instance.set_total_price()     instance.save()

current models

class orderitem(models.model):     order = models.foreignkey('main.order')     item = models.foreignkey('main.item')     quantity = models.integerfield(default=1)   class order(models.model):     user = models.foreignkey("main.shopuser")     items = models.manytomanyfield("main.item", through='main.orderitem')     placed = models.booleanfield(default=false)     date_placed = models.datetimefield(null=true, blank=true)

i feel silly not figuring out before. instead of m2m_changed signal use post_save , post_delete on through model , should same thing. in serializer remove instance.save() if not doing other changes. 

@receiver((post_save, post_delete), sender="main.orderitem") def update_order_when_items_changed(sender, instance, **kwargs):     instance.order.set_weight()     instance.order.set_total_price()     instance.order.save()

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