I see. I had feared that might be the answer when I was trying to think of
a way to implement it.
Incrementing the counter was just meant as a quick example not an actual
use case. Maybe firing off a small function where accuracy can be fudged a
bit when changes are detected.
Thanks for your response.
On Thursday, June 12, 2014 11:44:35 AM UTC-4, Tom Evans wrote:
>
> On Thu, Jun 12, 2014 at 3:48 PM, Mezzy >
> wrote:
> > Hello
> >
> > How do I detect when related models have changed; If I have models such
> as
> > the following:
> >
> > .. models.py
> > class Model1(models.Model):
> >
> > field = models.CharField(max_length=255, default="Some value")
> >
> >
> > class Model2(models.Model):
> >
> > field = models.CharField(max_length=255, default="Some value")
> >
> >
> > class Model3(models.Model):
> >
> > field = models.CharField(max_length=255, default="Some value")
> >
> >
> > class Model4(models.Model):
> >
> > one = models.OneToOneField(Model1)
> >
> > foreign = models.ForeignKey(Model2)
> >
> > many = models.ManyToManyField(Model3)
> >
> > counter = models.IntegerField(default=1)
> >
> >
> >
> > .. test.py
> > one = Model1.objects.create()
> > foreign = Model2.objects.create()
> > many = [Model3.objects.create() for i in xrange(0,5)]
> > x = Model2.objects.create(one=one, foreign=foreign)
> > x.many = many
> > x.save()
> >
> >
> >
> >
> >
> >
> > Then I perform the following operations:
> > one.field = "new value"
> > one.save()
> > foreign.field = "newer value"
> > foreign.save()
> > many[0].field = "newest value"
> > many[0].save()
> >
> >
> > How do I detect when these objects have changed? Is there a signal for
> this
> > type of thing?
> > For example if I want to increment Model4's counter field every time one
> of
> > these objects change, what should I do?
>
> There isn't such a signal because of the complexity (and
> impossibility, in some cases) of doing so.
>
> For instance, if a Model2 object is modified, there is potentially one
> Model1 instance to signal that a related object has changed, but if a
> Model3 or Model4 object is modified, potentially all Model1 instances
> might require to be signalled.
>
> There are signals for when a model instance itself changes; if you so
> wish you can hook in to that and add application specific logic to
> discover which Model1 instances are related to that changed instance,
> and fire events for each of them.
>
> Putting a counter on the Model1 is a denormalisation; using signals to
> keep a denormalisation up to date is fragile, especially when using
> django signals where there are multiple ways that data could change
> without a signal firing.
>
> Cheers
>
> Tom
>
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