Django: “Soft” ForeignField without database integrity checks

Question

I have a Django project that has multiple django \"apps\". One of them has models to represent data coming from an external source (I do not control this data).

I want m

Answer 1

I tried something similar to Izz ad-Din Ruhulessin's suggestion but it didn't work because I have columns other than the "fake FK" column. The code I tried was:

class DynamicPkg(models.Model):
    @property
    def cities(self):
        return City.objects.filter(dpdestinations__dynamic_pkg=self)


class DynamicPkgDestination(models.Model):
    dynamic_pkg = models.ForeignKey(DynamicPkg, related_name='destinations')
    # Indexed because we will be joining City.code to
    # DynamicPkgDestination.city_code and we want this to be fast.
    city_code = models.CharField(max_length=10, db_index=True)


class UnmanagedDynamicPkgDestination(models.Model):
    dynamic_pkg = models.ForeignKey(DynamicPkg, related_name='destinations')
    city = models.ForeignKey('City', db_column='city_code', to_field='code', related_name='dpdestinations')

    class Meta:
        managed = False
        db_table = DynamicPkgDestination._meta.db_table


class City(models.Model):
    code = models.CharField(max_length=10, unique=True)

and the errors I got were:

Error: One or more models did not validate:
travelbox.dynamicpkgdestination: Accessor for field 'dynamic_pkg' clashes with related field 'DynamicPkg.destinations'. Add a related_name argument to the definition for 'dynamic_pkg'.
travelbox.dynamicpkgdestination: Reverse query name for field 'dynamic_pkg' clashes with related field 'DynamicPkg.destinations'. Add a related_name argument to the definition for 'dynamic_pkg'.
travelbox.unmanageddynamicpkgdestination: Accessor for field 'dynamic_pkg' clashes with related field 'DynamicPkg.destinations'. Add a related_name argument to the definition for 'dynamic_pkg'.
travelbox.unmanageddynamicpkgdestination: Reverse query name for field 'dynamic_pkg' clashes with related field 'DynamicPkg.destinations'. Add a related_name argument to the definition for 'dynamic_pkg'.

However I did come up with a working solution by using a proxy model. I did still have to hack around some Django validation that prevents fields from being included in proxy models:

class DynamicPkg(models.Model):
    @property
    def cities(self):
        return City.objects.filter(dpdestinations__dynamic_pkg=self)



def proxify_model(new_class, base):
    """
    Like putting proxy = True in a model's Meta except it doesn't spoil your
    fun by raising an error if new_class contains model fields.
    """
    new_class._meta.proxy = True
    # Next 2 lines are what django.db.models.base.ModelBase.__new__ does when
    # proxy = True (after it has done its spoil-sport validation ;-)
    new_class._meta.setup_proxy(base)
    new_class._meta.concrete_model = base._meta.concrete_model


class DynamicPkgDestination(models.Model):
    dynamic_pkg = models.ForeignKey(DynamicPkg, related_name='destinations')
    # Indexed because we will be joining City.code to
    # DynamicPkgDestination.city_code and we want this to be fast.
    city_code = city_code_field(db_index=True)


class ProxyDynamicPkgDestination(DynamicPkgDestination):
    city = models.ForeignKey('City', db_column='city_code', to_field='code', related_name='dpdestinations')


proxify_model(ProxyDynamicPkgDestination, DynamicPkgDestination)


class City(models.Model):
    code = models.CharField(max_length=10, unique=True)