django-model-utils

Django model mixins and utilities.

Installation

Install from PyPI with pip:

pip install django-model-utils

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 supports Django 1.4.2 and later on Python 2.6, 2.7, 3.2, and 3.3.

Contributing

Please file bugs and send pull requests to the GitHub repository and issue tracker.

(Until January 2013 django-model-utils primary development was hosted at BitBucket; the issue tracker there will remain open until all issues and pull requests tracked in it are closed, but all new issues should be filed at GitHub.)

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).

StatusField does not set db_index=True automatically; if you expect to frequently filter on your status field (and it will have enough selectivity to make an index worthwhile) you may want to add this yourself.

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 different, 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()

InheritanceManager

This manager (contributed by Jeff Elmore) should be attached to a base model class in a model-inheritance tree. It allows queries on that base model to return heterogenous results of the actual proper subtypes, without any additional queries.

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 attach an InheritanceManager to Place, you can just call the select_subclasses() method on the InheritanceManager or any QuerySet from it, and the resulting objects will be instances of Restaurant or Bar:

from model_utils.managers import InheritanceManager

class Place(models.Model):
    # ...
    objects = InheritanceManager()

class Restaurant(Place):
    # ...

class Bar(Place):
    # ...

nearby_places = Place.objects.filter(location='here').select_subclasses()
for place in nearby_places:
    # "place" will automatically be an instance of Place, Restaurant, or Bar

The database query performed will have an extra join for each subclass; if you want to reduce the number of joins and you only need particular subclasses to be returned as their actual type, you can pass subclass names to select_subclasses(), much like the built-in select_related() method:

nearby_places = Place.objects.select_subclasses("restaurant")
# restaurants will be Restaurant instances, bars will still be Place instances

InheritanceManager also provides a subclass-fetching alternative to the get() method:

place = Place.objects.get_subclass(id=some_id)
# "place" will automatically be an instance of Place, Restaurant, or Bar

If you don't explicitly call select_subclasses() or get_subclass(), an InheritanceManager behaves identically to a normal Manager; so it's safe to use as your default manager for the model.

Note

Due to Django bug #16572, on Django versions prior to 1.6 InheritanceManager only supports a single level of model inheritance; it won't work for grandchild models.

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).

PassThroughManager

A common "gotcha" when defining methods on a custom manager class is that those same methods are not automatically also available on the QuerySets returned by that manager, so are not "chainable". This can be counterintuitive, as most of the public QuerySet API is mirrored 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 PassThroughManager class (contributed by Paul McLanahan) removes this boilerplate.

To use PassThroughManager, rather than defining a custom manager with additional methods, define a custom QuerySet subclass with the additional methods you want, and pass that QuerySet subclass to the PassThroughManager.for_queryset_class() class method. The returned PassThroughManager subclass will always return instances of your custom QuerySet, and you can also call methods of your custom QuerySet directly on the manager:

from datetime import datetime
from django.db import models
from django.db.models.query import QuerySet
from model_utils.managers import PassThroughManager

class PostQuerySet(QuerySet):
    def by_author(self, user):
        return self.filter(user=user)

    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 = PassThroughManager.for_queryset_class(PostQuerySet)()

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

FieldTracker

A FieldTracker can be added to a model to track changes in model fields. A FieldTracker allows querying for field changes since a model instance was last saved. An example of applying FieldTracker to a model:

from django.db import models
from model_utils import FieldTracker

class Post(models.Model):
    title = models.CharField(max_length=100)
    body = models.TextField()

    tracker = FieldTracker()

Note

django-model-utils 1.3.0 introduced the ModelTracker object for tracking changes to model field values. Unfortunately ModelTracker suffered from some serious flaws in its handling of ForeignKey fields, potentially resulting in many extra database queries if a ForeignKey field was tracked. In order to avoid breaking API backwards-compatibility, ModelTracker retains the previous behavior but is deprecated, and FieldTracker has been introduced to provide better ForeignKey handling. All uses of ModelTracker should be replaced by FieldTracker.

Summary of differences between ModelTracker and FieldTracker:

• The previous value returned for a tracked ForeignKey field will now be the raw ID rather than the full object (avoiding extra database queries). (GH-43)
• The changed() method no longer returns the empty dictionary for all unsaved instances; rather, None is considered to be the initial value of all fields if the model has never been saved, thus changed() on an unsaved instance will return a dictionary containing all fields whose current value is not None.
• The has_changed() method no longer crashes after an object's first save. (GH-53).

Accessing a field tracker

There are multiple methods available for checking for changes in model fields.

previous

Returns the value of the given field during the last save:

>>> a = Post.objects.create(title='First Post')
>>> a.title = 'Welcome'
>>> a.tracker.previous('title')
u'First Post'

Returns None when the model instance isn't saved yet.

has_changed

Returns True if the given field has changed since the last save:

>>> a = Post.objects.create(title='First Post')
>>> a.title = 'Welcome'
>>> a.tracker.has_changed('title')
True
>>> a.tracker.has_changed('body')
False

The has_changed method relies on previous to determine whether a field's values has changed.

changed

Returns a dictionary of all fields that have been changed since the last save and the values of the fields during the last save:

>>> a = Post.objects.create(title='First Post')
>>> a.title = 'Welcome'
>>> a.body = 'First post!'
>>> a.tracker.changed()
{'title': 'First Post', 'body': ''}

The changed method relies on has_changed to determine which fields have changed.

Tracking specific fields

A fields parameter can be given to FieldTracker to limit tracking to specific fields:

from django.db import models
from model_utils import FieldTracker

class Post(models.Model):
    title = models.CharField(max_length=100)
    body = models.TextField()

    title_tracker = FieldTracker(fields=['title'])

An example using the model specified above:

>>> a = Post.objects.create(title='First Post')
>>> a.body = 'First post!'
>>> a.title_tracker.changed()
{'title': None}