Abstract Model Mixins

Warning

This is just a stub document. It will be fleshed out more. If you wish to comment on it, please e-mail coreyoordt at gmail.

What problem does this pattern solve?

It creates a tool kit to build complex models.

When to use it

For models that you will commonly build in projects but have many different potential features, and you want your models to only contain the features necessary. Blogs are a good example, where where are many potential options to include within a blog, but you don’t need all of them all the time.

Why should I use it?

This allows developers to fix bugs once, in the tool kit. Installing the new tool kit version will fix those bugs in each model created from it.

Where is it used?

I first saw this in GLAMkit‘s blogtools package. It is also used in Django 1.3’s class-based views.

Implementation

Models

First isolate all the potential or optional features from core features. The core features and each isolated set of optional features will make up individual abstract Django models.

GLAMkit isolated the one base model (EntryBase), and four optional features: add a featured flag, add a status field, add a tag field, and allow for the body and excerpt content to convert to HTML.

EntryBase includes seven fields–author, title, pub_date, slug, enable_comments, excerpt, and body–as well as __unicode__(), get_absolute_url() and excerpt_or_body() functions. The Meta class has abstract=True so that Django never tries to represent this model in a database. It must be subclassed.

FeaturableEntryMixin is an abstract class that merely defines an is_featured field.

StatusableEntryMixin is an abstract class that defines LIVE_STATUS, DRAFT_STATUS, and HIDDEN_STATUS values and choices. It defines a status field with those choices.

TaggableEntryMixin is an abstract class that is only available if Django-Tagging is installed, as it uses the TagField for the tags field it defines.

HTMLFormattableEntryMixin is a much more complex abstract class. It is only available if the template_utils package is available. It defines two text fields, excerpt_html and body_html. It also overrides the save() method so it can convert the contents of exceprt and body into HTML for excerpt_html and body_html, respectively. Finally it re-defines the excerpt_or_body() method to return the excerpt_html or body_html value.

Admin

It is difficult to provide a really good ModelAdmin class when you aren’t sure what fields or features are included in the final model. GLAMkit provides a EntryAdminBase which is subclassed from object (not ModelAdmin). Providing other admin mixins would make sense if there were admin-specific features to provide, such as adding a WYSIWYG editor, autocomplete lookups or special filtering.

URLs

Views

Syndication Feeds

How to use it

Sources

Useful Links

GLAMKit (Gallery, Library, Archive, Museum) an Australian group, tackles this situation with a set of abstract classes that provide very basic features.

You create your model by subclassing the classes that provide the functionality you need.

And you don’t have to stop there. You can add your own fields as well.

class PRBlog(EntryBase,
             StatusableEntryMixin):

    subhead = models.CharField()
    pdf = models.FileField()