django-model-utils 0.5.0

Django model mixins and utilities

Django model mixins and utilities.

Installation

Install from PyPI with pip:

pip install django-model-utils

or get the in-development version:

pip install django-model-utils==tip

To use django-model-utils in your Django project, just import and use the utility classes described below; there is no need to modify your INSTALLED_APPS setting.

Dependencies

django-model-utils requires Django 1.0 or later.

Choices

Choices provides some conveniences for setting choices on a Django model field:

from model_utils import Choices

class Article(models.Model):
    STATUS = Choices('draft', 'published')
    # ...
    status = models.CharField(choices=STATUS, default=STATUS.draft, max_length=20)

A Choices object is initialized with any number of choices. In the simplest case, each choice is a string; that string will be used both as the database representation of the choice, and the human-readable representation. Note that you can access options as attributes on the Choices object: STATUS.draft.

But you may want your human-readable versions translated, in which case you need to separate the human-readable version from the DB representation. In this case you can provide choices as two-tuples:

from model_utils import Choices

class Article(models.Model):
    STATUS = Choices(('draft', _('draft')), ('published', _('published')))
    # ...
    status = models.CharField(choices=STATUS, default=STATUS.draft, max_length=20)

But what if your database representation of choices is constrained in a way that would hinder readability of your code? For instance, you may need to use an IntegerField rather than a CharField, or you may want the database to order the values in your field in some specific way. In this case, you can provide your choices as triples, where the first element is the database representation, the second is a valid Python identifier you will use in your code as a constant, and the third is the human-readable version:

from model_utils import Choices

class Article(models.Model):
    STATUS = Choices((0, 'draft', _('draft')), (1, 'published', _('published')))
    # ...
    status = models.IntegerField(choices=STATUS, default=STATUS.draft)

StatusField

A simple convenience for giving a model a set of “states.” StatusField is a CharField subclass that expects to find a STATUS class attribute on its model, and uses that as its choices. Also sets a default max_length of 100, and sets its default value to the first item in the STATUS choices:

from model_utils.fields import StatusField
from model_utils import Choices

class Article(models.Model):
    STATUS = Choices('draft', 'published')
    # ...
    status = StatusField()

(The STATUS class attribute does not have to be a Choices instance, it can be an ordinary list of two-tuples).

MonitorField

A DateTimeField subclass that monitors another field on the model, and updates itself to the current date-time whenever the monitored field changes:

from model_utils.fields import MonitorField, StatusField

class Article(models.Model):
    STATUS = Choices('draft', 'published')

    status = StatusField()
    status_changed = MonitorField(monitor='status')

(A MonitorField can monitor any type of field for changes, not only a StatusField.)

SplitField

A TextField subclass that automatically pulls an excerpt out of its content (based on a “split here” marker or a default number of initial paragraphs) and stores both its content and excerpt values in the database.

A SplitField is easy to add to any model definition:

from django.db import models
from model_utils.fields import SplitField

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

SplitField automatically creates an extra non-editable field _body_excerpt to store the excerpt. This field doesn’t need to be accessed directly; see below.

Accessing a SplitField on a model

When accessing an attribute of a model that was declared as a SplitField, a SplitText object is returned. The SplitText object has three attributes:

content:

The full field contents.

excerpt:

The excerpt of content (read-only).

has_more:

True if the excerpt and content are the same, False otherwise.

This object also has a __unicode__ method that returns the full content, allowing SplitField attributes to appear in templates without having to access content directly.

Assuming the Article model above:

>>> a = Article.objects.all()[0]
>>> a.body.content
u'some text\n\n<!-- split -->\n\nmore text'
>>> a.body.excerpt
u'some text\n'
>>> unicode(a.body)
u'some text\n\n<!-- split -->\n\nmore text'

Assignment to a.body is equivalent to assignment to a.body.content.

Note

a.body.excerpt is only updated when a.save() is called

Customized excerpting

By default, SplitField looks for the marker <!-- split --> alone on a line and takes everything before that marker as the excerpt. This marker can be customized by setting the SPLIT_MARKER setting.

If no marker is found in the content, the first two paragraphs (where paragraphs are blocks of text separated by a blank line) are taken to be the excerpt. This number can be customized by setting the SPLIT_DEFAULT_PARAGRAPHS setting.

TimeFramedModel

An abstract base class for any model that expresses a time-range. Adds start and end nullable DateTimeFields, and a timeframed manager that returns only objects for whom the current date-time lies within their time range.

StatusModel

Pulls together StatusField, MonitorField and QueryManager into an abstract base class for any model with a “status.”

Just provide a STATUS class-attribute (a Choices object or a list of two-tuples), and your model will have a status field with those choices, a status_changed field containing the date-time the status was last changed, and a manager for each status that returns objects with that status only:

from model_utils.models import StatusModel
from model_utils import Choices

class Article(StatusModel):
    STATUS = Choices('draft', 'published')

# ...

a = Article()
a.status = Article.STATUS.published

# this save will update a.status_changed
a.save()

# this query will only return published articles:
Article.published.all()

InheritanceCastModel

This abstract base class can be inherited by the root (parent) model in a model-inheritance tree. It allows each model in the tree to “know” what type it is (via an automatically-set foreign key to ContentType), allowing for automatic casting of a parent instance to its proper leaf (child) type.

For instance, if you have a Place model with subclasses Restaurant and Bar, you may want to query all Places:

nearby_places = Place.objects.filter(location='here')

But when you iterate over nearby_places, you’ll get only Place instances back, even for objects that are “really” Restaurant or Bar. If you have Place inherit from InheritanceCastModel, you can just call the cast() method on each Place and it will return an instance of the proper subtype, Restaurant or Bar:

from model_utils.models import InheritanceCastModel

class Place(InheritanceCastModel):
    # ...

class Restaurant(Place):
    # ...

nearby_places = Place.objects.filter(location='here')
for place in nearby_places:
    restaurant_or_bar = place.cast()
    # ...

Note

This is inefficient for large querysets, as it results in n queries to the subtype tables. It would be possible to write a QuerySet subclass that could reduce this to k queries, where there are k subtypes in the inheritance tree.

TimeStampedModel

This abstract base class just provides self-updating created and modified fields on any model that inherits from it.

QueryManager

Many custom model managers do nothing more than return a QuerySet that is filtered in some way. QueryManager allows you to express this pattern with a minimum of boilerplate:

from django.db import models
from model_utils.managers import QueryManager

class Post(models.Model):
    ...
    published = models.BooleanField()
    pub_date = models.DateField()
    ...

    objects = models.Manager()
    public = QueryManager(published=True).order_by('-pub_date')

The kwargs passed to QueryManager will be passed as-is to the QuerySet.filter() method. You can also pass a Q object to QueryManager to express more complex conditions. Note that you can set the ordering of the QuerySet returned by the QueryManager by chaining a call to .order_by() on the QueryManager (this is not required).

manager_from

A common “gotcha” when defining methods on a custom manager class is that those same methods are not automatically also available on the QuerySet used by that model, so are not “chainable”. This can be counterintuitive, as most of the public QuerySet API is also available on managers. It is possible to create a custom Manager that returns QuerySets that have the same additional methods, but this requires boilerplate code.

The manager_from function (created by George Sakkis and included here by permission) solves this problem with zero boilerplate. It creates and returns a Manager subclass with additional behavior defined by mixin subclasses or functions you pass it, and the returned Manager will return instances of a custom QuerySet with those same additional methods:

from datetime import datetime
from django.db import models

class AuthorMixin(object):
    def by_author(self, user):
        return self.filter(user=user)

class PublishedMixin(object):
    def published(self):
        return self.filter(published__lte=datetime.now())

def unpublished(self):
    return self.filter(published__gte=datetime.now())


class Post(models.Model):
    user = models.ForeignKey(User)
    published = models.DateTimeField()

    objects = manager_from(AuthorMixin, PublishedMixin, unpublished)

Post.objects.published()
Post.objects.by_author(user=request.user).unpublished()

CHANGES

tip (unreleased)

0.5.0 (2010.09.24)

• added manager_from (thanks George Sakkis)

• added StatusField, MonitorField, TimeFramedModel, and StatusModel (thanks Jannis Leidel)

• deprecated ChoiceEnum and replaced with Choices

0.4.0 (2010.03.16)

• added SplitField

• added ChoiceEnum

• added South support for custom model fields

0.3.0

• Added QueryManager

TODO

• Custom QuerySet subclass to pair with InheritanceCastModel for more efficient querying.