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 nearby_places = Place.objects.select_subclasses("restaurant", "bar") # all Places will be converted to Restaurant and Bar instances.
It is also possible to use the subclasses themselves as arguments to select_subclasses, leaving it to calculate the relationship for you:
nearby_places = Place.objects.select_subclasses(Restaurant) # restaurants will be Restaurant instances, bars will still be Place instances nearby_places = Place.objects.select_subclasses(Restaurant, Bar) # all Places will be converted to Restaurant and Bar instances.
It is even possible to mix and match the two:
nearby_places = Place.objects.select_subclasses(Restaurant, "bar") # all Places will be converted to Restaurant and Bar 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.
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.
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).
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()
Each of the above manager classes has a corresponding mixin that can be used to add functionality to any manager. For example, to create a GeoDjango GeoManager that includes “pass through” functionality, you can write the following code:
from django.contrib.gis.db import models from django.contrib.gis.db.models.query import GeoQuerySet from model_utils.managers import PassThroughManagerMixin class PassThroughGeoManager(PassThroughManagerMixin, models.GeoManager): pass class LocationQuerySet(GeoQuerySet): def within_boundary(self, geom): return self.filter(point__within=geom) def public(self): return self.filter(public=True) class Location(models.Model): point = models.PointField() public = models.BooleanField(default=True) objects = PassThroughGeoManager.for_queryset_class(LocationQuerySet)() Location.objects.public() Location.objects.within_boundary(geom=geom) Location.objects.within_boundary(geom=geom).public()
Now you have a “pass through manager” that can also take advantage of GeoDjango’s spatial lookups. You can similarly add additional functionality to any manager by composing that manager with InheritanceManagerMixin or QueryManagerMixin.
(Note that any manager class using InheritanceManagerMixin must return a QuerySet class using InheritanceQuerySetMixin from its get_queryset method. This means that if composing InheritanceManagerMixin and PassThroughManagerMixin, the QuerySet class passed to PassThroughManager.for_queryset_class must inherit InheritanceQuerySetMixin.)