Related objects reference¶
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
ForeignKeyrelation. That is:
from django.db import models class Reporter(models.Model): # ... pass class Article(models.Model): reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
In the above example, the methods below will be available on the manager
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
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.
You can use the
bulk=Falseargument to instead have the related manager perform the update by calling
add()with a many-to-many relationship, however, will not call any
save()methods, but rather create the relationships using
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 parameter
create(). Django figures out that the new
blogfield 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
QuerySet.delete()which means no model
save()methods are called; listen to the
m2m_changedsignal if you wish to execute custom code when a relationship is deleted.
ForeignKeyobjects, this method only exists if
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 the
null=True, this is invalid.
ForeignKeyobjects, this method accepts a
bulkargument to control how to perform the operation. If
QuerySet.update()is used. If
save()method of each individual model instance is called instead. This triggers the
post_savesignals 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=Trueand it also accepts the
set(objs, bulk=True, clear=False)¶
Replace the set of related objects:
>>> new_list = [obj1, obj2, obj3] >>> e.related_set.set(new_list)
This method accepts a
clearargument to control how to perform the operation. If
False(the default), the elements missing from the new set are removed using
remove()and only the new ones are added. If
clear()method is called instead and the whole set is added at once.
bulkargument is passed on to
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 to
clear()and the call to
set()all 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 relationship.
Also, if you are using an intermediate model for a many-to-many relationship, then the
set()methods are disabled.
If you use
set()methods clear the prefetched cache.Changed in Django 1.11:
The clearing of the prefetched cache described above was added.
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 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.
Deprecated since version 1.10: Direct assignment is deprecated in favor of the
>>> e.related_set.set([obj1, obj2, obj3])
This prevents confusion about an assignment resulting in an implicit save.