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(Almost) Single table polymorphism in Django

Author:
julkiewicz
Posted:
April 10, 2011
Language:
Python
Version:
1.3
Score:
1 (after 3 ratings)

An emulation of "table per hierarchy" a.k.a. "single table inheritance" in Django. The base class must hold all the fields. It's subclasses are not allowed to contain any additional fields and optimally they should be proxies. They however may provide additional methods to operate on the declared field. The presented solution supports implicit inheritance, even across ForeignKeys, and ManyToManyFields. No additional database hits are imposed.

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from abc import abstractmethod
from django.db import models
from django.db.models import query


class PolymorphicQuerySet(query.QuerySet):
    
    def get(self, *args, **kwargs):
        if kwargs.has_key('model_class'):
            cls = kwargs.pop('model_class')
            kwargs['mod'] = cls.__module__
            kwargs['cls'] = cls.__name__
        return super(PolymorphicQuerySet, self).get(*args, **kwargs)
    
    def filter(self, *args, **kwargs):
        if kwargs.has_key('model_class'):
            cls = kwargs.pop('model_class')
            kwargs['mod'] = cls.__module__
            kwargs['cls'] = cls.__name__
        return super(PolymorphicQuerySet, self).filter(*args, **kwargs)


class PolymorphicManager(models.Manager):
    
    def get_query_set(self):
        return PolymorphicQuerySet(self.model, using=self._db)
    
    def get(self, *args, **kwargs):
        return self.get_query_set().get(*args, **kwargs)
    
    def filter(self, *args, **kwargs):
        return self.get_query_set().filter(*args, **kwargs)


class PolymorphicModel(models.Model):
    
    classes = dict()
    mod = models.CharField(max_length=50)
    cls = models.CharField(max_length=30)
    
    class Meta:
        abstract = True
    
    @staticmethod
    def __new__(cls, *args, **kwargs):
        if len(args) > len(cls._meta.fields):
            raise IndexError("Number of args exceeds number of fields")
        c = None
        m = None
        fields_iter = iter(cls._meta.fields)
        for val, field in itertools.izip(args, fields_iter):
            if field.name == 'cls':
                c = val
            elif field.name == 'mod':
                m = val
        if c is None:
            c = kwargs.get('cls', None)
        if m is None:
            m = kwargs.get('mod', None)
        if c is not None:
            assert m is not None
            m = str(m)
            c = str(c)
            if not Model.classes.has_key(c):
                cls = getattr(__import__(m, globals(), locals(), [c]), c)
                PolymorphicModel.classes[(m, c)] = cls
            else:
                cls = Model.classes[(m, c)]
        return super(PolymorphicModel, cls).__new__(cls, *args, **kwargs)
    
    def save(self, *args, **kwargs):
        if not self.cls:
            self.mod = self.__class__.__module__
            self.cls = self.__class__.__name__
            super(PolymorphicModel, self).save(*args, **kwargs)


class SomeBase(PolymorphicModel):

    a = IntegerField(default=0)
    b = IntegerField(default=0)
    
    @abstractmethod
    def something(self): pass


class SomeDerivedA(SomeBase):
    
    class Meta:
        proxy = True

    def something(self):
        self.a += 1


class SomeDerivedB(SomeBase):
    
    class Meta:
        proxy = True

    def something(self):
        self.b += 1


class SomeCollection(models.Model):
    
    values = models.ManyToManyField(SomeBase)


>>> a = SomeDerivedA()
>>> a.save()
>>> b = SomeDerivedB()
>>> b.save()
>>> x = SomeCollection()
>>> x.values = [a, b]
>>> x.save()

>>> y = SomeCollection.objects.get(pk=x.pk)
>>> y.values.objects[0].something()
>>> y.values.objects[0].a
1
>>> y.values.objects[0].b
0
>>> y.values.objects[1].something()
>>> y.values.objects[1].a
0
>>> y.values.objects[1].b
1

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Comments

julkiewicz (on September 22, 2011):

Corrected a serious bug - using new instead of init to perform class changes.

#

facundo_olano (on October 25, 2011):

I think there are some imports missing, one is itertools, the other is Model; the second I don't know where to import from.

#

patseng (on July 10, 2013):

Any suggestions on how to resolve his:

  • NameError: global name 'Model' is not defined

@facundo_olano already mentioned it. Is there a solution?

#

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