Wednesday, May 25, 2011

Django Models Mixins

One thing I've been experimenting with is model Mixins.  For example, the aim is to create small abstract classes that are each focused around a particular function.  These abstract classes can then be added to arbitrary models to apply those functions to models as desired.

For example, say I define a RatingsFields abstract class and a TrackingFields abstract class.  These abstract classes can be mixed into any other model that we wish to add rating or tracking functionality to.

core/mixins.py
from djangoratings.fields import RatingField # 3rd party module

class RatingFields(models.Model):
    
    rating = RatingField(range=5) # 5 possible rating values, 1-5

    class Meta:
        abstract = True
        

class TrackingFields(models.Model):

    deleted_on = models.DateTimeField(blank=True, null=True)
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True
        

Since we applied the abstract classes to the Post model below, the model now has rating and tracking capabilities.  This is useful to help simplify code where a lot of models share fields or methods with the same function.

myapp/models.py
from core import mixins as core_mixins
class Post(core_mixins.TrackingFields, core_mixins.RatingFields):
    name = models.CharField(max_length=128)
    slug = models.SlugField(max_length=128)
    ...

Comments welcome.
Joe

8 comments:

  1. Did you try django.contrib.contenttypes?

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  2. Thanks for your comment, Pavel. I've used content types and generic relations in the past and they are very useful.

    With this mixin approach, the fields are created on the given table rather than linked through a foreign key. This might be handy for situations where we know we are going to need the same fields on a given table - such as the TrackingFields - and want to avoid code duplication. Granted, this might reduce some of the flexibility that a generic relation would afford us, but perhaps also reduce some of the complexity.

    If you have other thoughts or suggestions regarding this, please feel free to share. I'm eager to learn of better approaches. Thank you again,
    Joe

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  3. Thanks Eduardo... core_interfaces is a copy-paste error. I've made the correction. Thank you for pointing that out.

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  4. Although this is a good example of using mixins, I wouldn't call this an example of aspect-orientated programming.

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  5. Thanks cody-somerville. Perhaps I was off on my naming. I'll adjust the title name.

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  6. I was trying to figure out the best way to do this (see http://stackoverflow.com/questions/6428075/is-it-ok-to-use-multiple-inheritance-with-django-abstract-models) , whether my Mixins needed to inherit from models.Model or not, and so forth. This is a good anwser.

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  7. Name and slug are very generic too, you could put them in a mixin...

    ReplyDelete