Value signals

[jochenklar]

This PR is a small spin-of from #1188, because I would like to have some feedback on the idea.

The PR adds 3 custom django signals value_created, value_deleted, and value_updated, which are fired when Values are created, updated or deleted through the interface. The front-end provides the API with the page the user in on, and this can be used in a handler to get all attributes on the current page like this:

@receiver(value_created, sender=Value)
def value_created_handler(sender, request, instance, **kwargs):
    page = Page.objects.filter(id=request.GET.get('page')).first()
    print('value_created', instance, page)


@receiver(value_updated, sender=Value)
def value_updated_handler(sender, request, instance, **kwargs):
    page = Page.objects.filter(id=request.GET.get('page')).first()
    print('value_updated', instance, page)


@receiver(value_deleted, sender=Value)
def value_deleted_handler(sender, request, instance, **kwargs):
    page = Page.objects.filter(id=request.GET.get('page')).first()
    print('value_deleted', instance, page)

The main benefit is that plugins/custom django apps could use them to manipulate answers on the same page without complicated queries.

[MyPyDavid]

could it give race conditions when I click a lot of checkboxes or another user is simultaneously clicking the same checkboxes?
For your example I guess not right, since it only gets the page ??

[jochenklar]

The signal is fired when the value is actually saved, so no additional race conditions should emerge (I think). The value itself is in instance in my example. If a value is changed by a handler and somebody else wants to edit the same value, the same ValidationError would happen as if two people would change values at the same time.