multi-level foreignkey throws KeyError

[Gopal Narayanan <gopastro@…>]

Example models:

class person(models.Model):
    pid = models.CharField(maxlength=20,
                           primary_key=True)
    name = models.CharField(maxlength=100)

class student(models.Model):
    student_pid = models.ForeignKey(person,
                                    db_column='student_pid',
                                    to_field='pid')

class student_option(models.Model):
    pid = models.ForeignKey(student,
                            to_field='student_pid')
    junk = models.IntegerField()

Note that student_option pid refers to student_pid which itself is a ForeignKey on student pointing to person.
Trying to generate SQL for these models throws KeyError: 'ForeignKey'

It appears that FKs are not allowed to point at a datatype that is itself a FK. It would be nice to allow multi-level foreign keys as there is a benefit in modeling legacy data with this kind of schema.

The patch attached simply enables the creation of SQL necessary for this to happen, by recursively diving down until we find the real underlying datatype.

[Chris Beaven]

slightly simpler patch

[Chris Beaven]

Although this is an obscure use-case, I validated that this does raise the above exception and that the patch fixes this.

[Malcolm Tredinnick]

(In [5106]) Fixed #2608 -- Generate correct SQL for multi-level foreign key relations.
Based on a patch from Gopal Narayanan.