Am Montag, 17. November 2014 16:25:54 UTC+1 schrieb Michael Bayer:
>
>
> On Nov 17, 2014, at 7:37 AM, Guido Winkelmann <
> gu...@ambient-entertainment.de <javascript:>> wrote:
>
> Hi,
>
> How can I go about having a cross-platform compatible compound unique
> constraint over two columns where there can be only one record with NULL in
> one column and a given value in the other?
>
> I want something like this:
>
> UniqueConstraint("parent_id", "name")
>
> except "parent_id" is nullable, and I want to disallow creating multiple
> records with parent_id=NULL and the same name. The problem here is that
> some database management systems, PostgreSQL for example, will treat all
> NULL values as not-equal inside a unique constraint, which makes sense for
> a single column unique, but not really for a multi-column one. This will
> allow multiple records with the same name and parent_id=NULL again.
>
> In PostgreSQL, apparently I would need to create a unique index in
> addition to the constraint. I would really like to solve this without
> writing database-specific code, though.
>
> Does anybody have a good solution for that?
>
>
> I’ve had an issue in this area before and decided just to forego it; I was
> targeting PG and SQL Server, but I don’t remember exactly which NULL
> behavior I was looking for. per the SQL standard I think PG’s behavior is
> correct. NULL means, “unknown”, so two NULL values are never equivalent.
>
If I wanted to add a manual check in Python for that, how would I go about
that?
Could I register an event handler for the "before_insert" event for this
model?
What would the event handler have to do to refuse an insert? Throw an
exception?
Guido W.
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