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POIs there any more elegant way to add a value sensitive unique together constraint in Django Model?
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2013-05-10T03:06:31.327
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Here is the problem: I have a model like this: <pre><code>class UserBook(models.Model): user = models.ForeignKey(User) book = models.ForeignKey(Book) is_active = models.BooleanField(default=False) class Meta: unique_together = ("user", "book") </code></pre> Obviously, this model already has a unique together constraint for field user and book. And probably there will be some entries like this in the database: <pre><code> ------------------------------ |user_id book_id is_active | | 1 1 0 | | 1 2 0 | | 1 3 1 | ------------------------------ </code></pre> And I have one more constraint to add, which is each user can have at most one entry that the value of is_active field is 1(True). Currently I solve this problem by changing the model into this: <pre><code>class UserBook(models.Model): user = models.ForeignKey(User) book = models.ForeignKey(Book) is_active = models.BooleanField(default=False) key = models.charFeild(max_length=255, unique=True) class Meta: unique_together = ("user", "book") def save(self, *args, **kwargs): if self.is_active: self.key = "%s_%s" %(self.user_id, self.is_active) else: self.key = "%s_%s_%s" %(self.user_id, self.is_active, self.book_id) </code></pre> Add a field key, and customize the save method of this model. But the max_length cannot be greater than 255 in this approach(which is no need to worry in my case, but sometimes the key field may be very long). So, I would like to know if there is any more elegant approach to solve this kind of problem. Thanks!
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<python><django><django-models><django-database>
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Is there any more elegant way to add a value sensitive unique together constraint in Django Model?
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