On Oct 27, 2:49 pm, Victor Hooi <victorh...@gmail.com> wrote:
> heya,
>
> In my models.py, I've got an "Address", as well as an abstract "Trip"
> object, and then a "ToTrip", and "ReturnTrip" object that derive from
> that. ToTrip and ReturnTrip both contain references to two Address
> objects (models.ForeignKey).
>
> class Address(models.Model):
> description = models.CharField(max_length=20, help_text='Helpful
> description of this location. Maximum 20 characters.')
> ...(other fields and methods omitted)
>
> class ToTrip(Trip):
> origin = models.ForeignKey(Address)
> destination = models.CharField(max_length=20, choices=CAMPUSES)
>
> # Todo: Is there a cleaner way of doing this?
> def __unicode__(self):
> return str(self.origin) + ' to ' + str(self.destination)
>
> class ReturnTrip(Trip):
> origin = models.CharField(max_length=20, choices=CAMPUSES)
> destination = models.ForeignKey(Address)
>
> # Todo: Is there a cleaner way of doing this?
> def __unicode__(self):
> return str(self.origin) + ' to ' + str(self.destination)
>
> In Address, there's a __unicode__() that returns a nicely formatted
> string.
>
> My first question is, for ToTrip and ReturnTrip, I wanted to create a
> a __unicode__ method that just returned a description field inside of
> address. My attempts at using:
>
> return self.origin.description
>
> failed, and I'm guessing that's not the right way to access a field
> through a FK relationship? In the above, I'm just using the
> __unicode__ method on each address to get it - but I'm sure there's a
> cleaner way to get access to the related object's fields themselves?
>
> My second question is a little trivial - is there a way to count the
> total number of Trip objects, or objects that are inheritting from it?
> At the moment, I'm just counting ReturnTrip and ToTrip objects, and
> adding them. No biggie, but I was wondering if there was a way to get
> the total number of Trip objects, or at least the number of children?
> (I'm guessing the number of actual Trip objects themselves would be
> zero?)
>
> Cheers,
> Victor
OK, firstly, you are returning bytestrings from the unicode methods,
instead of (obviously) unicode strings. This will eventually bite you:
if there's ever a value that contains a non-ASCII character (eg a
foreign address), it will die horribly. You should not be using str()
in your unicode methods.
So, for example, in ToTrip, the method should look like this:
def __unicode__(self):
return u'%s to %s' % (self.origin, self.destination)
Now, in terms of your actual question, there's certainly nothing wrong
with doing 'return self.origin.description' - that certainly is the
way to access related fields. What happens when you try? Can you post
the exact traceback?
Of course this will fail when the object does not have the foreign key
field set - eg when it's newly created. Perhaps this is the problem
you're experiencing. You should catch that possibility and return a
sensible default.
On your second question, this depends on whether Trip is an abstract
class or not. If so, there are indeed no Trip objects to count. If
not, ie you are using multi-table inheritance, you can use
Trip.objects.count() to get the total number.
--
DR.
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