ForeignKey validation should check that to_field is unique

[physicsnick]

Currently, ForeignKey validation does not check that to_field is unique. Here is an example:

class Author(models.Model): 
    code = models.CharField(max_length=10, db_index=True) 
    first_name = models.CharField(max_length=30) 
    last_name = models.CharField(max_length=40) 
class Book(models.Model): 
    title = models.CharField(max_length=100) 
    author = models.ForeignKey(Author, to_field='code')

Django happily accepts these models. For PostgresSQL and Oracle, foreign keys must be unique, but MySQL and SQLite make no such requirement. MySQL requires only that the column be indexed, and SQLite has no requirements (it does not enforce foreign key constraints).

Django requires uniqueness so it should be enforcing this on its own. The documentation of to_field should also specify that it should be unique.

Here is the relevant MySQL documentation:

​http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html

...MySQL and InnoDB require that the referenced columns be indexed for performance. However, the system does not enforce a requirement that the referenced columns be UNIQUE or be declared NOT NULL.

Here is some extended discussion:
​http://groups.google.com/group/django-users/browse_thread/thread/fcd3915a19ae333e

[Marcos Moyano]

django.db.fields.related patch

[Marcos Moyano]

A humble approach at related field initialization. Don't know how to write a test suite for this since it's on initialization. I've ran the test suite just to make sure nothing breaks.

[Marcos Moyano]

New patch

[Marcos Moyano]

New patch. Moved validation code to django.core.management.validation and add test suite.

I think the validation could be clearear and simpler if we add a line like this:
self.to_field = to_field
in django.db.models.fields.related on ForeignKey init method.

[Karen Tracey]

It's not fixed until patch is committed.

[Karen Tracey]

(In [12756]) Fixed #11702: Catch to_field specifying a non-unique target in validation. Thanks marcosmoyano.

[Jacob]

Milestone 1.2 deleted

[anandjeyahar1]

Shouldn't this fix be specific based on the db being used? i.e syncdb should check the current db being used and raise an error only for postgresql or oracle. Am using mysql and i shouldn't be blocked from using a non_unique field as a foreign key.

[Karen Tracey]

No. The validation is needed especially for databases that don't enforce this constraint. Django's ForeignKey is many-to-one, the Django code relies on that characteristic, and you can get odd results if the database does not enforce it, see for example this thread:

​https://groups.google.com/forum/?fromgroups#!searchin/django-users/QuerySet.count%28%29$20inaccurate$20across$20ForeignKey$20relationships/django-users/fMmWVDr0Re8/overview