Model mixins

Model mixins are abstract classes that can be added as a parent class of a model. Python supports multiple inheritances, unlike other languages such as Java. Hence, you can list any number of parent classes for a model.

Mixins ought to be orthogonal and easily composable. Drop in a mixin to the list of base classes and they should work. In this regard, they are more similar in behavior to composition rather than inheritance.

Smaller mixins are better. Whenever a mixin becomes large and violates the single responsibility principle, consider refactoring it into smaller classes. Let a mixin do one thing and do it well.

In our previous example, the model mixin used to update created and modified time can be easily factored out, as shown in the following code:

class TimeStampedModel(models.Model): 
    created = models.DateTimeField(auto_now_add=True) 
    modified = models.DateTimeField(auto_now =True) 
 
    class Meta: 
        abstract = True 
 
class Postable(TimeStampedModel): 
    message = models.TextField(max_length=500) 
    ...  
 
    class Meta: 
        abstract = True 
 
class Post(Postable): 
    ... 
 
class Comment(Postable): 
    ... 

We have two base classes now. However, the functionality is clearly separated. The mixin can be separated into its own module and reused in other contexts.