Related objects reference¶
-
class
RelatedManager
¶ 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
ForeignKey
relation. That is:from django.db import models class Blog(models.Model): # ... pass class Entry(models.Model): blog = models.ForeignKey(Blog, on_delete=models.CASCADE, null=True)
In the above example, the methods below will be available on the manager
blog.entry_set
.Both sides of a
ManyToManyField
relation: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 onpizza.toppings
.
-
add
(*objs, bulk=True, through_defaults=None)¶ Adds the specified model objects to the related object set.
Example:
>>> 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
ForeignKey
relationship,QuerySet.update()
is used to perform the update. This requires the objects to already be saved.You can use the
bulk=False
argument to instead have the related manager perform the update by callinge.save()
.Using
add()
with a many-to-many relationship, however, will not call anysave()
methods (thebulk
argument doesn’t exist), but rather create the relationships usingQuerySet.bulk_create()
. If you need to execute some custom logic when a relationship is created, listen to them2m_changed
signal, which will triggerpre_add
andpost_add
actions.Using
add()
on a relation that already exists won’t duplicate the relation, but it will still trigger signals.For many-to-many relationships
add()
accepts either model instances or field values, normally primary keys, as the*objs
argument.Use the
through_defaults
argument to specify values for the new intermediate model instance(s), if needed. You can use callables as values in thethrough_defaults
dictionary and they will be evaluated once before creating any intermediate instance(s).
-
create
(through_defaults=None, **kwargs)¶
-
acreate
(through_defaults=None, **kwargs)¶ Asynchronous version:
acreate
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 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 parameter
blog
tocreate()
. Django figures out that the newEntry
object’sblog
field should be set tob
.Use the
through_defaults
argument to specify values for the new intermediate model instance, if needed. You can use callables as values in thethrough_defaults
dictionary.Changed in Django 4.1:acreate()
method was added.
-
remove
(*objs, bulk=True)¶ 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.
Similar to
add()
,e.save()
is called in the example above to perform the update. Usingremove()
with a many-to-many relationship, however, will delete the relationships usingQuerySet.delete()
which means no modelsave()
methods are called; listen to them2m_changed
signal if you wish to execute custom code when a relationship is deleted.For many-to-many relationships
remove()
accepts either model instances or field values, normally primary keys, as the*objs
argument.For
ForeignKey
objects, this method only exists ifnull=True
. If the related field can’t be set toNone
(NULL
), then an object can’t be removed from a relation without being added to another. In the above example, removinge
fromb.entry_set()
is equivalent to doinge.blog = None
, and because theblog
ForeignKey
doesn’t havenull=True
, this is invalid.For
ForeignKey
objects, this method accepts abulk
argument to control how to perform the operation. IfTrue
(the default),QuerySet.update()
is used. Ifbulk=False
, thesave()
method of each individual model instance is called instead. This triggers thepre_save
andpost_save
signals and comes at the expense of performance.For many-to-many relationships, the
bulk
keyword argument doesn’t exist.
-
clear
(bulk=True)¶ 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.
Just like
remove()
,clear()
is only available onForeignKey
s wherenull=True
and it also accepts thebulk
keyword argument.For many-to-many relationships, the
bulk
keyword argument doesn’t exist.
-
set
(objs, bulk=True, clear=False, through_defaults=None)¶ Replace the set of related objects:
>>> new_list = [obj1, obj2, obj3] >>> e.related_set.set(new_list)
This method accepts a
clear
argument to control how to perform the operation. IfFalse
(the default), the elements missing from the new set are removed usingremove()
and only the new ones are added. Ifclear=True
, theclear()
method is called instead and the whole set is added at once.For
ForeignKey
objects, thebulk
argument is passed on toadd()
andremove()
.For many-to-many relationships, the
bulk
keyword argument doesn’t exist.Note that since
set()
is a compound operation, it is subject to race conditions. For instance, new objects may be added to the database in between the call toclear()
and the call toadd()
.For many-to-many relationships
set()
accepts a list of either model instances or field values, normally primary keys, as theobjs
argument.Use the
through_defaults
argument to specify values for the new intermediate model instance(s), if needed. You can use callables as values in thethrough_defaults
dictionary and they will be evaluated once before creating any intermediate instance(s).
Note
Note that
add()
,create()
,remove()
,clear()
, andset()
all apply database changes immediately for all types of related fields. In other words, there is no need to callsave()
on either end of the relationship.If you use
prefetch_related()
, theadd()
,remove()
,clear()
, andset()
methods clear the prefetched cache.
Django 4.1
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