On Sun, Oct 15, 2006 at 01:26:18PM -0700, MerMer wrote:
> I have two Models.
>
> 1. Product
> 2. Review - There can be many reviews for any product (one -to many
> relationship)
>
> One of the fields of Review is "Rating". This holds an integer between
> 1 and 5.
>
> I want to display a list of Products, and display the average review
> rating for each product.
Other responses have pointed you in the direction of custom
SQL. There's another (less data-normalized) way, which can be
implemented without SQL knowledge: use signals. This also has the
benefit of being easily sortable.
Step one is to add an "average rating" attribute to your Product
model. Set it to default to 0.0, or allow nulls and default to null.
Step two is to create a function (a function, not an object method)
which will update the average rating. That function should take this
argument list:
(sender, instance, signal, *args, **kwargs)
"instance" is the important one here, as it will be a Review
object. Use that to get the Product object, and then calculate the
average rating, set the avg_rating attribute of that Product object,
and call save().
Step three is wiring that function to the signal you
want. Specifically, you probably want the pre_save or post_save
signal, fired from the Review model.
You can do all of this in your models.py, which would end up looking
something like this:
# a bunch of imports
from django.db.models import signals
from django.dispatch import dispatcher
class Product:
# blah blah blah
class Review:
# blah blah blah
def update_average_rating(sender, instance, signal, *args, **kwargs):
# 'instance' will be the Review object being saved
#
# use that to get the Product object affected, calculate the new
# average rating, then call .save() on the Product object
dispatcher.connect(update_average_rating, signal=signals.pre_save,
sender=Review)
Once you do that, the "average rating" attribute in your Product model
will be updated automatically every time a new Review is created (or
when an old Review is modified).
Do a google search for "django signals" for more information.
-johnnnnnnnn
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