Setting up a foreign key to an abstract base class with Django


I've factored out common attributes from two classes into an abstract base class, however I have another model that needs to reference either one of those classes. It's not possible to reference an ABC as it doesn't actually have a database table.

The following example should illustrate my problem:

class Answer(models.Model):
    ovramt = models.ForeignKey("Ovramt")
    question = models.ForeignKey("Question")
    answer = models.CharField(max_length=3, choices=(("yes","yes"),("no","no") ("NA","N/A"))
    likelihood = models.IntegerField(choices=LIKELY_CHOICES)
    consequence = models.IntegerField(choices=CONSEQUENCE_CHOICES)

    class Meta:
        abstract = True

class Answer_A(Answer):
    resident = models.ForeignKey("Resident")
    def __unicode__(self):
        return u"%s - %s - %s" %(, self.resident, self.question)    

class Answer_B(Answer):
    def __unicode__(self):
        return u"%s - %s" %(, self.question)    

class Answer_Risk(models.Model):
    answer = models.ForeignKey("Answer")
    risk = models.CharField(max_length=200)

    def __unicode__(self):
        return self.risk

Answer_A and Answer_B are slightly different in that Answer_A also needs a FK relationship to another table. Answer_B may also require some specific attributes later. The problem would STILL exist if I had Answer_B be the superclass - and have Answer_A subclass or compose it.

A 'Risk' is the same whether it's Answer_A or Answer_B. I also have other models that need to reference an 'Answer' regardless of it's sub-type. How can this be done? How can you reference a type regardless of it's sub-type?

I was trying to avoid a join operation but I don't think I'm going to be able to. Would it be worth having the reference to 'Resident' in all 'Answer's and just nulling it where required? Or is that considered very bad practice?

1/12/2009 7:36:56 AM

Accepted Answer

A generic relation seems to be the solution. But it will complicate things even further.

It seems to me; your model structure is already more complex than necessary. I would simply merge all three Answer models into one. This way:

  • Answer_Risk would work without modification.
  • You can set resident to None (NULL) in case of an Answer_A.
  • You can return different string represantations depending on resident == None. (in other words; same functionality)

One more thing; are your answers likely to have more than one risk? If they'll have none or one risk you should consider following alternative implementations:

My main concern is neither database structure nor performance (although these changes should improve performance) but code maintainability.

1/6/2012 6:15:12 AM

My gut would be to suggest removing the abstract modifier on the base class. You'll get the same model structure, but Answer will be it's own table. The downside of this is that if these are large tables and/or your queries are complex, queries against it could be noticeably slower.

Alternatively, you could keep your models as is, but replace the ForeignKey to Answer with a GenericForeignKey. What you lose in the syntactic sugar of model inheritance, you gain a bit in query speed.

I don't believe it's possible to reference an abstract base model by ForeignKey (or anything functionally the same).

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