Related objects reference¶
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
ForeignKeyrelation. 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
Both sides of a
class Topping(models.Model): ... class Pizza(models.Model): toppings = models.ManyToManyField(Topping)
In this example, the methods below will be available both on
These related managers have some extra methods:
add(obj1[, obj2, ...])¶
Adds the specified model objects to the related object set.
>>> b = Blog.objects.get(id=1) >>> e = Entry.objects.get(id=234) >>> b.entry_set.add(e) # Associates Entry e with Blog b.
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
create(). Django figures out that the new
blogfield should be set to
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
null=True. If the related field can’t be set to
NULL), then an object can’t be removed from a relation without being added to another. In the above example, removing
b.entry_set()is equivalent to doing
e.blog = None, and because the
null=True, this is invalid.