Django: save multiple object signal once

Question

Hello, I need some help with sending email when an order is placed. To illustrate the problem, following is the abstract code:

class Order(models.Model):
    user = models.ForeignKey(User)

class OrderItem(modes.Model):
    order = models.ForeignKey(Order, related_name='items')
    item = models.CharField(max_length=255)
    unit_price = models.DecimalField()
    qty = models.IntegerField()
    item_amount = models.DecimalField()

def email_order_on_save(sender, instance, **kwargs):
    # Need order.items.all() here
    pass

post_save.connect(email_order_on_save, sender=Order)

Most of the problems on SO and google seem to deal with one child object at a time; such as this.

Listening to OrderItem would release 5 signals if 5 orders items saved from admin inlines. I can't seem to get my head around this problem. One way, I think (not sure if possible), could be listening to last of all(5) OrderItem's post_save signals.

Any help appreciated.

Answer 1

I'm guessing you're trying to solve this in the wrong place. Sending an email when the order is completed and saving the Order model are at different levels of abstraction.

I think sending the email should be triggered by some condition in the view that has more information about whether the order is completely saved or not. Think for example of what will happen if an order needs updating (say it's status changes)? Should the email be sent then too?

Answer 2

Create your own custom signal and send it at the point when you have the data you need saved. Pass in as parameters whatever data structures you need.

Listen for your custom signal in your callback function email_order_on_save and make appropriate decisions based on the parameters about sending or not the e-mail.

Answer 3

I think you can have a problem with signals, OrderItem with inlines will not send save signal, read this

Answer 4

You could create your model as follows

ORDER_STATE = (
    (1, 'Completed'),
    (2, 'Processing'),
)

class Order(models.Model):
    user = models.ForeignKey(User)
    state = models.IntegerField(choices = ORDER_STATE)

You could have many states for the order. The state "Completed" could represent that the order processing is complete. You could change the state of your order in your views.

In the signal handler, you could check for the state of the order and then send mail, if the order is in completed state.