Re: Using unique_together with foreign key fields which may be None

On Mon, Sep 30, 2013 at 4:55 PM, Berndt Jung <berndt@brkt.com> wrote:

Because the lookup_value of a null field is None, the validation check is aborted entirely.  This seems wrong to me, and I'm wondering if I'm doing something wrong here.  Redefining the models is not something I can do at this point.

None in Python corresponds to NULL in SQL. SQL defines that NULL != NULL. So when checking for unique values the database will not consider two NULLs to be the same value; the NULLs will be considered to be different and will not violate the unique constraint. The Django code is doing the same check for uniquess that the database will be enforcing. There is no way I know of to make a database enforce uniqueness among NULL values...

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