关系对象参考

关系对象参考

class RelatedManager

“关系管理器”是一个用于处理“一对多”和“多对多”关联关系的管理器。在以下两种情况用到：

“另一边”的关联关系 ForeignKey。即:

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

在上面的例子中，以下方法会在管理器 reporter.article_set 中可用。

双边关系 ManyToManyField

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

在这个例子中，下文列出的方法在 topping.pizza_set 和 pizza.toppings 中均可用。

add(\objs, bulk=True, through_defaults=None*)

将特定的模型对象加入关联对象集合。

举例:
```
>>> b = Blog.objects.get(id=1)
>>> e = Entry.objects.get(id=234)
>>> b.entry_set.add(e) # Associates Entry e with Blog b.
```
上述例子中，由于有 ForeignKey 关联关系，才可以使用 QuerySet.update() 更新数据。这要求对象事先已经被保存在数据库内了。

你可以使用 bulk=False 参数让关系管理器通过调用 e.save() 来执行更新操作。

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.

For many-to-many relationships add() accepts either model instances or field values, normally primary keys, as the *objs argument.

Use the through_defaults argument to specify values for the new intermediate model instance(s), if needed. You can use callables as values in the through_defaults dictionary and they will be evaluated once before creating any intermediate instance(s).

Changed in Django 3.1:

through_defaults values can now be callables.
create(through_defaults=None, \*kwargs*)

创建一个新对象，保存至数据库，并将其放入关联对象集合内。返回刚创建的对象:
```
>>> 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 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)
```
无需指定定义了模型间关系的关键字参数。在上述例子中，我们并未向 create() 传递参数 blog。Django 知道要将新 Entry 对象的 blog 字段设置为 b。

Use the through_defaults argument to specify values for the new intermediate model instance, if needed. You can use callables as values in the through_defaults dictionary.

Changed in Django 3.1:

through_defaults values can now be callables.
remove(\objs, bulk=True*)

从关系集合中移除特定的对象:
```
>>> 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 many-to-many relationships remove() accepts either model instances or field values, normally primary keys, as the *objs argument.

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.

For many-to-many relationships, the bulk keyword argument doesn’t exist.
clear(bulk=True)

清空关系集合:
```
>>> 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.

For many-to-many relationships, the bulk keyword argument doesn’t exist.
set(objs, bulk=True, clear=False, through_defaults=None)

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.

For ForeignKey objects, the bulk argument is passed on to add() and remove().

For many-to-many relationships, the bulk keyword argument doesn’t exist.

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().

For many-to-many relationships set() accepts a list of either model instances or field values, normally primary keys, as the objs argument.

Use the through_defaults argument to specify values for the new intermediate model instance(s), if needed. You can use callables as values in the through_defaults dictionary and they will be evaluated once before creating any intermediate instance(s).

Changed in Django 3.1:

through_defaults values can now be callables.

注解

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.

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