non-null FK to a CharField primary key fails when key value is the empty string, giving "[model.fk_col] may not be NULL"

[Andrew Badr]

Model A has a CharField primary_key. Model B has a ForeignKey field to model A. If an instance of A has a row with an empty-string in the primary key column, no B instance can point to it. Attempting to create such a B instance fails with "[model.fk_col] may not be NULL". Repro'd with PG and sqlite3.

[Andrew Badr]

Seems to be caused by ForeignKey.get_db_prep_save (called from Model.save_base):

    def get_db_prep_save(self, value):
        if value == '' or value == None:
            return None
        ...

Unsure where to go since I don't know the reasoning behind this code. Is this due to forms assigning the empty-string to fields left blank?

[syko]

Replying to andrewbadr:

Model A has a CharField primary_key. Model B has a ForeignKey field to model A. If an instance of A has a row with an empty-string in the primary key column, no B instance can point to it. Attempting to create such a B instance fails with "[model.fk_col] may not be NULL". Repro'd with PG and sqlite3.

I encountered a similar problem in django 1.2.1 using MySQL, but I don't think it has anything to do with foreign keys. The documentation says "Note that empty string values will always get stored as empty strings, not as NULL", which is clearly not the case. Assigning a empty ("") string value to a CharField(null=False, blank=True) and saving the model will throw a "column cannot be NULL" error.
Thus the only way to store empty values in CharFields is having null=True.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Anssi Kääriäinen]

This should be fixed at some point. The fix should involve making sure '' works as expected in other parts of Django, too.

[Tim Graham]

Duplicate of #19299; fixed in Django 1.7.