QuerySet.update improvement.

[Sebastian Noack]

Hi,

I think QuerySet.update works to inflexible. In SQL you can define
expressions containing the current value or the value of an other
column in the update clause, for Example to incrementing a value in the
result set. This is not possible by the ORM at the moment.

I developed a possible solution, by introducing combinable Expression
objects. Look at http://code.djangoproject.com/ticket/7210.

This patch allows you to do following:

        from django.db.models.sql.expressions import *

        qset = model.objects.all()

        # Equivalent to qset.update(foo=42)
        qset.update(foo=LiteralExpr(42))
        # Increment column 'foo' by one.
        qset.update(foo=CurrentExpr() + LiteralExpr(1))
        # Swap the value of the column 'foo' and 'bar'.
        qset.update(foo=ColumnExpr('bar'), bar=ColumnExpr('foo'))

Does somebody thinks there are reasons to don't merge this patch? Or
ideas ti even improve this patch? Thanks.

Regards
Sebastian Noack

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