On Thu, Oct 7, 2010 at 4:54 AM, meenakshi <meenak...@mbi.ucla.edu> wrote:
> Hi,
> I am new to Django and Python and have been working through the
> tutorial. I would like to modify the polls app in the tutorial such
> that I can track individual users who vote in a poll, keep a record of
> their choice, and not allow any user to vote more than once in any
> given poll.
> Briefly, there are many polls, each poll being linked to several
> choices. Each user can vote in many polls, but is allowed to vote
> only once in each individual poll. Whats the best way to enforce
> this?
> I tried to enforce this by using the unique_together feature to bind a
> specific user, poll, choice combination together (within the Choice
> class). It didnt work. Here is the code I tried:
>
> from django.db import models
> import datetime
>
> class User(models.Model):
> name = models.CharField(max_length=100)
> email = models.EmailField()
> def __unicode__(self):
> return self.name
>
> class Poll(models.Model):
> question = models.CharField(max_length=200)
> pub_date = models.DateTimeField('date published')
> def __unicode__(self):
> return self.question
>
> class Choice(models.Model):
> poll = models.ForeignKey(Poll)
> user = models.ManyToManyField(User)
> choice = models.CharField(max_length=200)
> votes = models.IntegerField()
>
> def __unicode__(self):
> return self.choice
>
> class Meta:
> unique_together = ("poll", "user", "choice")
>
>
> My error message:
> Error: One or more models did not validate:
> polls.choice: "unique_together" refers to user. ManyToManyFields are
> not supported in unique_together.
> polls.choice: "unique_together" refers to user. This is not in the
> same model as the unique_together statement.
>
> I would appreciate any suggestions/corrections,
>
> Thanks,
> Meenakshi
>
Make your model heirarchy such that adding a vote is modelled by the
creation of a single row in a single table, and the creation of that
row is constrained by the DB to respect your conditions (1 vote per
user per poll):
class User(models.Model):
pass
class Poll(models.Model):
pass
class Choice(models.Model):
poll = models.ForeignKey(Poll)
class UserPollChoice(models.Model):
poll = models.ForeignKey(Poll)
user = models.ForeignKey(User)
choice = models.ForeignKey(Choice)
class Meta:
unique_together = ( 'poll', 'user', )
You will have to use aggregates to get the scores of the poll out with
this scenario though. Something like this would work:
from django.db.models import Count
poll.choice_set.annotate(num_votes=Count('userpollchoice'))
Cheers
Tom
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