related objects passed to model init are not cached/accessible

[Gary Wilson]

Models:

from django.db import models

class Color(models.Model):
    name = models.CharField(max_length=10)

class Knight(models.Model):
    favorite = models.ForeignKey(Color)

Neither saved or unsaved related objects get cached:

# Unsaved object.
>>> k = Knight(favorite=Color())
>>> hasattr(k, '_favorite_cache')
False

# Saved object.
>>> k = Knight(favorite=Color.objects.create(name='blue'))
>>> hasattr(k, '_favorite_cache')
False

# But assignment with saved or unsaved objects does cache correctly.
>>> k = Knight()
>>> k.favorite = Color()
>>> hasattr(k, '_favorite_cache')
True
>>> k = Knight()
>>> k.favorite = Color.objects.create(name='blue')
>>> hasattr(k, '_favorite_cache')
True

Also, in the saved related object case above, we do an extra query to refetch the object by id.

There's also a problem when trying to access the field when it was passed an unsaved object in the init:

# Unsaved object.
>>> k = Knight(favorite=Color())
>>> k.favorite
Traceback (most recent call last):
...
DoesNotExist

# Saved object.
>>> k = Knight(favorite=Color.objects.create(name='blue'))
>>> k.favorite
<Color: blue>

The attached patch does both:

• Caches saved or unsaved related object passed to the Model init.
• Allows unsaved related objects to be accessed when they were passed to the Model init.

[Gary Wilson]

corrected a comment

[Gary Wilson]

Someone else care to review?

[Gary Wilson]

Patch note: #6886 made the type checking happen on assignment, so we don't need to do the checking in the init anymore as long as you are setting using the attribute named by field.name.

[Gary Wilson]

(In [8185]) Fixed #8070 -- Cache related objects passed to Model init as keyword arguments. Also:

• Model init no longer performs a database query to refetch the related objects it is passed.
• Model init now caches unsaved related objects correctly, too. (Previously, accessing the field would raise DoesNotExist error for null=False fields.)
• Added tests for assigning None to null=True ForeignKey fields (refs #6886).

[Jacob]

Milestone 1.0 deleted

[Aymeric Augustin]

Allows unsaved related objects to be accessed when they were passed to the Model init.

Django accepted this but the relation wasn't saved, see #10811. I'm going to remove the test for that bug-prone behavior.