# Τεκμηρίωση του Django - 1.10.7

# 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 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 `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`.

  `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 `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 calling `e.save()`.

    Using `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 `m2m_changed` signal.

    The `bulk` parameter was added. In older versions, foreign key updates were always done using `save()`. Use `bulk=False` if you require the old behavior.

  `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`.

  `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 `ForeignKey`s 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.

    The `bulk` argument is passed on to `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()`.

  Σημείωση

  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.

## Direct Assignment

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.

In earlier versions, direct assignment used to perform `clear()` followed by `add()`. It now performs a `set()` with the keyword argument `clear=False`.

Αποσύρθηκε στην έκδοση 1.10: Direct assignment is deprecated in favor of the `set()` method:

    >>> e.related_set.set([obj1, obj2, obj3])

This prevents confusion about an assignment resulting in an implicit save.