A “related manager” is a manager used in a one-to-many or many-to-many related context. This happens in two cases:
The “other side” of a
from django.db import models class Reporter(models.Model): # ... pass class Article(models.Model): reporter = models.ForeignKey(Reporter)
In the above example, the methods below will be available on
Both sides of a
class Topping(models.Model): # ... pass class Pizza(models.Model): toppings = models.ManyToManyField(Topping)
In this example, the methods below will be available both on
topping.pizza_set and on
Adds the specified model objects to the related object set.
>>> b = Blog.objects.get(id=1) >>> e = Entry.objects.get(id=234) >>> b.entry_set.add(e) # Associates Entry e with Blog b.
In the example above, in the case of a
e.save() is called by the related manager to perform the update.
add() with a many-to-many relationship, however, will not
save() methods, but rather create the relationships
QuerySet.bulk_create(). If you need to execute
some custom logic when a relationship is created, listen to the
Creates a new object, saves it and puts it in the related object set. Returns the newly created object:
>>> b = Blog.objects.get(id=1) >>> e = b.entry_set.create( ... headline='Hello', ... body_text='Hi', ... pub_date=datetime.date(2005, 1, 1) ... ) # No need to call e.save() at this point -- it's already been saved.
This is equivalent to (but much simpler than):
>>> b = Blog.objects.get(id=1) >>> e = Entry( ... blog=b, ... headline='Hello', ... body_text='Hi', ... pub_date=datetime.date(2005, 1, 1) ... ) >>> e.save(force_insert=True)
Note that there’s no need to specify the keyword argument of the model
that defines the relationship. In the above example, we don’t pass the
create(). Django figures out that the new
blog field should be set to
Removes the specified model objects from the related object set:
>>> b = Blog.objects.get(id=1) >>> e = Entry.objects.get(id=234) >>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
e.save() is called in the example above
to perform the update. Using
remove() with a many-to-many
relationship, however, will delete the relationships using
means no model
save() methods are called; listen to the
m2m_changed signal if you wish to
execute custom code when a relationship is deleted.
ForeignKey objects, this method only
null=True. If the related field can’t be set to
NULL), then an object can’t be removed from a relation without
being added to another. In the above example, removing
b.entry_set() is equivalent to doing
e.blog = None, and because
ForeignKey doesn’t have
null=True, this is invalid.
ForeignKey objects, this method accepts
bulk argument to control how to perform the operation.
True (the default),
QuerySet.update() is used.
save() method of each individual model
instance is called instead. This triggers the
post_save signals and comes at the
expense of performance.
Removes all objects from the related object set:
>>> b = Blog.objects.get(id=1) >>> b.entry_set.clear()
Note this doesn’t delete the related objects – it just disassociates them.
clear() is only available on
null=True and it also
bulk keyword argument.
apply database changes immediately for all types of related fields. In
other words, there is no need to call
save() on either end of the
Also, if you are using an intermediate model for a many-to-many relationship, some of the related manager’s methods are disabled.
A related object set can be replaced in bulk with one operation by assigning a new iterable of objects to it:
>>> new_list = [obj1, obj2, obj3] >>> e.related_set = new_list
If the foreign key relationship has
null=True, then the related manager
will first call
clear() to disassociate any existing objects in the related
set before adding the contents of
new_list. Otherwise the objects in
new_list will be added to the existing related object set.