Enforcing unique combinations of fields in Django

django, django-models, foreign-key-relationship

Solution

You got it, `unique_together` is the way to do this. It's as simple as:

class MyModelItem(models.Model): 
    ...
    name = models.CharField(max_length=100)
    model = models.ForeignKey(MyModel, related_name='items')
    item_type = models.CharField(max_length=1, choices=TYPE_CHOICES)

    class Meta:
        unique_together = ('model', 'item_type')

Problem

There is a model which could have multiple items linked to it, like: ``` from django.db import models class MyModel(models.Model): name = models.CharField(max_length=100) def __unicode__(self): return self.name class MyModelItem(models.Model): TYPE_CHOICES = ( ('A', 'A NAME'), ('B', 'B NAME'), ('C', 'C NAME') ) name = models.CharField(max_length=100) model = models.ForeignKey(MyModel, related_name='items') item_type = models.CharField(max_length=1, choices=TYPE_CHOICES) def __unicode__(self): return self.name ``` The requirement is: for each record of `MyModel`, don't allow more than one record of `MyModelItem` which has the same `item_type`, which means if I have: ``` from myproj.myapp.models import MyModel, MyModelItem m1 = MyModel.objects.create(name='M1') MyModelItem.objects.create(name='Item1', model=m1, item_type='A') ``` then I can't add another record with the same type 'A' to `m1`. One solution for me is doing it like this within `MyModelItem`: ``` ... def save(self, *args, **kwargs): if self.model.items.filter(item_type=self.item_type): raise IntegrityError("%s already have a item with type %s" % (self.model, self.item_type)) super(MyModelItem, self).save(*args, **kwargs) ``` but this won't protect the limitation on the db level. It's possible to add such not-allowed records with SQL directly, and it can break my application. Is there an approach to implement this requirement in Django? Something like `unique_together`?

Original source