Session.query(MyModel).filter_by(foo=old_foo_value).update({'foo':
new_foo_value})
This generates something like this: UPDATE mymodel SET foo=%s WHERE
mymodel.foo=%s
If I read that correctly it means that the update won't set any rows if the
value foo has changed in some other transaction since I last read it, so I
won't end up overwriting anything that's been changed by someone else.
If I use .all() and make my changes on the instances, the generated sql
issues will update where the primary key matches, rather than 'foo': UPDATE
mymodel SET foo=%s WHERE mymodel.uuid=%s
On Thursday, April 24, 2014 2:03:19 AM UTC+1, Michael Bayer wrote:
>
> perhaps I’m missing something but wouldn’t you just change the update()
> here to all(), so that you SELECT only those rows you care about into
> memory, then change each “foo” as needed and flush? I’m not seeing what
> the issue is. The row isn’t locked if you aren’t using SELECT..FOR UPDATE.
>
>
> On Apr 23, 2014, at 8:27 PM, Tim Kersten <[email protected] <javascript:>>
> wrote:
>
> I'd like to run a data migration on a live server, but only update rows if
> the data hasn't changed since I've read it, and would like to do so
> optimistically, so without locking the row. Doing so like below works and
> prevents me updating rows that have changed since I last read the row.
>
> Session.query(MyModel).filter_by(foo=old_foo_value).filter_by(bar=old_bar_value).update({'foo':
>
> new_foo_value})
>
> While the approach works, it doesn't use the ORM. (I use an after_flush
> hook to inspect & log changes from dirty instances in the session).
>
> Is there a way to update an ORM instance conditionally like above?
>
> Kindest Regards,
> Tim
>
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