How to use dynamic foreignkey in Django?

Here is how I do it:

from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import fields


class Photo(models.Model):
    picture = models.ImageField(null=True, upload_to='./images/')
    caption = models.CharField(_("Optional caption"),max_length=100,null=True, blank=True)

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = fields.GenericForeignKey('content_type', 'object_id')

class Article(models.Model):
    ....
    images     = fields.GenericRelation(Photo)

You would add something like

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = fields.GenericForeignKey('content_type', 'object_id')

to Faves and

    fields.GenericRelation(Faves)

to Article and Cast

contenttypes docs

Here's an approach. (Note that the models are singular, Django automatically pluralizes for you.)

class Article(models.Model):
    title = models.CharField(max_length=100)
    body = models.TextField()

class Cast(models.Model):
    title = models.CharField(max_length=100)
    body = models.TextField()

FAVE_CHOICES = ( 
    ('A','Article'),
    ('C','Cast'),
)
class Fave(models.Model):
    type_of_fave = models.CharField( max_length=1, choices=FAVE_CHOICES )
    cast = models.ForeignKey(Casts,null=True)
    article= models.ForeigKey(Articles,null=True)
    user = models.ForeignKey(User,unique=True)

This rarely presents profound problems. It may require some clever class methods, depending on your use cases.