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:

• "other side" dari sebuah hubungan ForeignKey. Yaitu:

  from django.db import models
  
  class Reporter(models.Model):
      # ...
      pass
  
  class Article(models.Model):
      reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)
  

  Dalam contoh diatas, metode-metode dibawah akan tersedia pada pengelola reporter.article_set.

• Kedua sisi dari hubungan ManyToManyField:

  class Topping(models.Model):
      # ...
      pass
  
  class Pizza(models.Model):
      toppings = models.ManyToManyField(Topping)
  

  Dalam contoh ini, metode-metode dibawah akan tersedia kedua pada topping.pizza_set dan pada pizza.toppings.

add(*objs, bulk=True)

Menambahkan obyek model ditentukan ke kumpulan obyek terkait.

Contoh:

>>> 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.

Anda dapat menggunakan argumen bulk=False daripada memiliki pengelola terkait melakukan pembaharuan dengan memanggil e.save().

Using add() with a many-to-many relationship, however, will not call any save() methods (the bulk argument doesn't exist), 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 m2m_changed signal, which will trigger pre_add and post_add actions.

Using add() on a relation that already exists won't duplicate the relation, but it will still trigger signals.

create(**kwargs)

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.

Ini adalah setara pada (tetapi lebih sederhana dari):

>>> 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 to create(). Django figures out that the new Entry object's blog field should be set to b.

remove(*objs)

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. 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_changed signal if you wish to execute custom code when a relationship is deleted.

For ForeignKey objects, this method only exists if null=True. If the related field can't be set to None (NULL), then an object can't be removed from a relation without being added to another. In the above example, removing e from b.entry_set() is equivalent to doing e.blog = None, and because the blog ForeignKey doesn't have null=True, this is invalid.

For ForeignKey objects, this method accepts a bulk argument to control how to perform the operation. If True (the default), QuerySet.update() is used. If bulk=False, the save() method of each individual model instance is called instead. This triggers the pre_save and post_save signals and comes at the expense of performance.

clear()

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 on ForeignKeys where null=True and it also accepts the bulk keyword argument.

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 clear argument 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=True, the clear() method is called instead and the whole set is added at once.

Argumen bulk dilewatkan pada add().

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 add().

Catatan

Note that add(), create(), remove(), clear(), and 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 add(), create(), remove(), and set() methods are disabled.

If you use prefetch_related(), the add(), remove(), clear(), and set() methods clear the prefetched cache.

Changed in Django 1.11:

The clearing of the prefetched cache described above was added.