class Like(models.Model):
    TARGET_MODEL = 'TargetModel'
    user = models.ForeignKey(settings.AUTH_USER_MODEL)
    target = models.ForeignKey(TARGET_MODEL)

    class Meta:
        abstract = True

class LikeForPost(Like):
    TARGET_MODEL = 'blog.Post'

class Like(models.Model):
    TARGET_MODEL = None  # will be overridden in subclass
    user = models.ForeignKey(settings.AUTH_USER_MODEL)

    @classmethod
    def on_class_prepared(cls):
        target_field = models.ForeignKey(cls.TARGET_MODEL)
        target_field.contribute_to_class(cls, 'target')

    class Meta:
        abstract = True

class LikeForPost(Like):
    TARGET_MODEL = 'blog.Post'

from django.apps import AppConfig
from django.db.models import signals

def call_on_class_prepared(sender, **kwargs):
    """Calls the function only if it is defined in the class being prepared"""
    try:
        sender.on_class_prepared()
    except AttributeError:
        pass


class BlogConfig(AppConfig):
    name = 'blog'

    def __init__(self, app_name, app_module):
        super(BlogConfig, self).__init__(app_name, app_module)
        # Connect programmatic class adjustment function to the signal
        signals.class_prepared.connect(call_on_class_prepared)

default_app_config = 'blog.apps.BlogConfig'