Model Managers¶
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 heterogeneous 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.
JoinManager¶
The JoinManager
will create a temporary table of your current queryset
and join that temporary table with the model of your current queryset. This can
be advantageous if you have to page through your entire DB and using django’s
slice mechanism to do that. LIMIT .. OFFSET ..
becomes slower the bigger
offset you use.
sliced_qs = Place.objects.all()[2000:2010]
qs = sliced_qs.join()
# qs contains 10 objects, and there will be a much smaller performance hit
# for paging through all of first 2000 objects.
Alternatively, you can give it a queryset and the manager will create a temporary
table and join that to your current queryset. This can work as a more performant
alternative to using django’s __in
as described in the following
(StackExchange answer).
big_qs = Restaurant.objects.filter(menu='vegetarian')
qs = Country.objects.filter(country_code='SE').join(big_qs)
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).
SoftDeletableManager¶
Returns only model instances that have the is_removed
field set
to False. Uses SoftDeletableQuerySet
, which ensures model instances
won’t be removed in bulk, but they will be marked as removed instead.
Mixins¶
Each of the above manager classes has a corresponding mixin that can be used to add functionality to any manager.
Note that any manager class using InheritanceManagerMixin
must return a
QuerySet
class using InheritanceQuerySetMixin
from its get_queryset
method.