========================= Related objects reference ========================= .. currentmodule:: django.db.models.fields.related .. 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 :class:`~django.db.models.ForeignKey` relation. 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 ``reporter.article_set``. * Both sides of a :class:`~django.db.models.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 on ``pizza.toppings``. .. method:: add(*objs, bulk=True) 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 :class:`~django.db.models.ForeignKey` relationship, :meth:`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 calling ``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 :meth:`QuerySet.bulk_create() `. If you need to execute some custom logic when a relationship is created, listen to the :data:`~django.db.models.signals.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. .. method:: 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. 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 ``blog`` to ``create()``. Django figures out that the new ``Entry`` object's ``blog`` field should be set to ``b``. .. method:: 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 :meth:`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 :meth:`QuerySet.delete()` which means no model ``save()`` methods are called; listen to the :data:`~django.db.models.signals.m2m_changed` signal if you wish to execute custom code when a relationship is deleted. For :class:`~django.db.models.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`` :class:`~django.db.models.ForeignKey` doesn't have ``null=True``, this is invalid. For :class:`~django.db.models.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 :data:`~django.db.models.signals.pre_save` and :data:`~django.db.models.signals.post_save` signals and comes at the expense of performance. .. method:: 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 :class:`~django.db.models.ForeignKey`\s where ``null=True`` and it also accepts the ``bulk`` keyword argument. .. method:: 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. The ``bulk`` argument is passed on to :meth:`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()``. .. note:: 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 :ref:`an intermediate model ` for a many-to-many relationship, then the ``add()``, ``create()``, ``remove()``, and ``set()`` methods are disabled. If you use :meth:`~django.db.models.query.QuerySet.prefetch_related`, the ``add()``, ``remove()``, ``clear()``, and ``set()`` methods clear the prefetched cache.