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:
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 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:
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.
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
(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.
In order to prevent database inconsistency, this method only exists on
ForeignKey
objects where 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.
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
.
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options9月 16, 2017