| ========================= |
| 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:: |
| |
| class Reporter(models.Model): |
| ... |
| |
| class Article(models.Model): |
| reporter = models.ForeignKey(Reporter) |
| |
| 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): |
| ... |
| |
| 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``. |
| |
| These related managers have some extra methods: |
| |
| .. method:: add(obj1, [obj2, ...]) |
| |
| 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, ``e.save()`` is called to perform the update. |
| Using ``add()`` with a many-to-many relationship, however, will not |
| call any ``save()`` methods, but rather create the relationships |
| using :meth:`QuerySet.bulk_create() |
| <django.db.models.query.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. |
| |
| .. 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(obj1, [obj2, ...]) |
| |
| 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()<django.db.models.query.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. |
| |
| .. 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``. |