Thanks Malcolm, on a lot of different levels.
1st, your code for putting the ratings into a list was very cool. It
provoked a light bulb moment in my brain as I worked my way through
the code. Very cool.
You're right about not being able to do the sum on the ratings, but I
fixed that by changing the code to:
ratings = [float(o.rating) for o in self.tastingnote_set.all()]
You're absolutely right about reading more about reverse relations.
The challenge was not even knowing to look up reverse relations so I
could read more about it.
Thanks again, not only for your thoughtful reply but also for pointing
me to where to look stuff up in the online docs.
Hani
On Oct 27, 9:41 pm, Malcolm Tredinnick <[EMAIL PROTECTED]>
wrote:
> On Sun, 2007-10-28 at 01:15 +0000, Hani wrote:
> > Hi,
> > A complete and utter newbie seeks help.
>
> > I have a model Beer with another model called TastingNotes tied via
> > foreign key to the Beer table.
>
> > models.py
>
> > class Beer(models.Model):
> > beer=models.CharField(max_length=50, core=True)
> > def __unicode__(self):
> > return self.beer
>
> > class TastingNote(models.Model):
> > beer=models.ForeignKey(Beer, edit_inline=models.TABULAR,
> > raw_id_admin=True, num_in_admin=1)
> > tastingnotes=models.TextField('Tasting notes', core=True)
> > rating=models.CharField(max_length=1,
> > choices=RATING_CHOICES,core=True)
>
> > def __unicode__(self):
> > return self.tastingnotes[:15]+"... Rating:"+self.rating
>
> > (note that RATING_CHOICES is defined at the top of models.py).
>
> > What I would like is to have a field, average_rating, that belongs to
> > the Beer model that is the average rating for that beer derived from
> > all the ratings in the TastingNote model. So I could call it like
> > this: beer.id.average_rating.
>
> > Would I do that with a manager class?
>
> A manager class is usually only necessary if you need a method that
> operates on *all* of the instances of this model. So, for example, if
> you wanted some function that operated on all the Beers, it would be an
> appropriate choice for a manager method on the Beer class.
>
> In this case, though, you have a specific beer that is the centre of
> attention, so just put a method on the Beer model. If you wrote
> something like this:
>
> def average_rating(self):
> ratings = [o.rating for o in self.tastingnote_set.all()]
> return sum(ratings) / len(ratings)
>
> as a method of the Beer model, it would do what you want. Then you could
> call it as beer.average_rating() in Python code or beer.average_rating
> in a template.
>
> [Except there's a problem here: You've defined the "rating" field on
> TastingNote to be a CharField, so "average" doesn't make a lot of sense.
> Do you really want it to be an IntegerField? None of what I wrote in
> that code will work for characters, because it assumes numerical
> properties exist. sum() doesn't do anything for strings and division
> certainly won't work.]
>
> If you aren't familiar with Python, the first line above might be a
> little confusing. Look up "list comprehensions" in the Python
> documentation.
>
> Also, from the sample code you wrote, it looks like you'll need to read
> the Django documentation for how to use reverse relations. If you have
> Beer instance and want to refer to all the TastingNote instances
> associated with it, you would use
>
> my_beer.tastingnote_set.all()
>
> like I did above (except that I had access to 'self' there, so that was
> the thing I used, not my_beer). See [1] for details on this (that's a
> link to deep in the middle of some documentation, so you might need to
> scroll up the beginning of the section for it to make sense and try out
> a few experiments).
>
> [1]http://www.djangoproject.com/documentation/db-api/#backward
>
> Regards,
> Malcolm
>
> --
> Despite the cost of living, have you noticed how popular it
> remains?http://www.pointy-stick.com/blog/
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