GenericForeignKeys in Intermediate Models

By : Derp

I'm attempting to create an intermediate model, Permissions, between 'auth.Group' and any other custom models; this will serve as permissions or a means of what is visible to which groups.

I have been able to create an intermediate model, ExamplePermissions, between 'auth.Group' and one model.

    class Example(TimeStampable, Ownable, Model):
        groups = models.ManyToManyField('auth.Group', through='ExamplePermissions', related_name='examples')
        name = models.CharField(max_length=255)
        # Used for chaining/mixins
        objects = ExampleQuerySet.as_manager()

        def __str__(self):

    class ExamplePermissions(Model):
        example = models.ForeignKey(Example, related_name='group_details')
        group = models.ForeignKey('auth.Group', related_name='example_details')
        write_access = models.BooleanField(default=False)

        def __str__(self):
            return ("{0}'s Example {1}").format(str(, str(self.example))

However, the issue is that this opposes reusability. To create a model that allows any custom model to be associated with it, I implemented a GenericForeignKey in place of a ForeignKey as follows:

    class Dumby(Model):
        groups = models.ManyToManyField('auth.Group', through='core.Permissions', related_name='dumbies')
        name = models.CharField(max_length=255)

        def __str__(self):

    class Permissions(Model):
        # Used to generically relate a model with the group model
        content_type = models.ForeignKey(ContentType, related_name='group_details')
        object_id = models.PositiveIntegerField()
        content_object = GenericForeignKey('content_type', 'object_id')
        group = models.ForeignKey('auth.Group', related_name='content_details')
        write_access = models.BooleanField(default=False)

        def __str__(self):
            return ("{0}'s Content {1}".format(str(, str(self.content_object)))

Upon attempting to make migrations, it errors with:
core.Permissions: (fields.E336) The model is used as an intermediate model by 'simulations.Dumby.groups', but it does not have a foreign key to 'Dumby' or 'Group'.

At first glance, using a GenericForeignKey in an intermediate table seems like a dead end. If this is the case, is there some generally accepted way of handling such a situation besides the cumbersome and redundant approach of creating a custom intermediate model for each custom model?

By : Derp


Do not use ManyToManyField when using GenericForeignKey in your intermediate model; instead, use GenericRelation, so your groups field would be simply declared as:

groups = generic.GenericRelation(Permissions)

See reverse generic relations for more details.

By : blhsing

awk to the rescue!

$ echo "a;b;\nXPTO;d" | awk -F';' '{print $1 "=" $3}'


or with file in/out

$ awk ... input_file > output_file
By : karakfa

The key word that you're apparently missing is "epsilon." If you search on Python epsilon value then it returns a links to StackOverflow: Value for epsilon in Python near the top of the results in Google.

Obviously it helps if you understand how the term is associated with this concept.

This video can help you solving your question :)
By: admin