Hi,
On Feb 10, 2010, at 6:54 PM, David Ressman wrote:
> Hello,
>
> First of all, I'll apologize if this is a really basic question. I've not
> been using SQLAlchemy for long, and I've only very recently picked up Python.
> Even though I've looked everywhere I can think of for an answer to this
> question, I'm almost certain that it is not a difficult one. I'm happy to
> read through any documentation you can point me to, but I've not been able to
> see what might be relevant to this particular question.
> ...
> What I want is an easy way to access a user's n most recent Usage objects for
> each filesystem. (For the purpose of this e-mail, we can take n=1.) It would
> be easy enough for me to just take the last m records in the
> user_obj.usage_data list (where m is the number of filesystems for which this
> user has records), but that's not really what I want. One filesystem might be
> storing records every hour, and another might be storing them every day. In
> that case, it would be hard to know how many records I would need to take
> from user_obj.usage_data to have the most recent record from each filesystem.
>
> It wouldn't be hard to actually use a session object to build a query for
> this, but I'd really like to have this all taken care of in the
> objects/maps/relations/whatever themselves. I'd like to have some attribute
> in the User object that's like user_obj.usage_data, but instead gives me a
> list of only the most recent Usage object from each filesystem, so I'd see
> something like:
>
>>>> user_obj.most_recent_usage
> [<Usage('user', 'fs1', 'some-date', foo:bar)>,
> <Usage('user', 'fs2', 'some-other-date', f0o:bAr)>,
> <Usage('user', 'fs3', 'some-third-date', fo0:b4r)>]
>
> I hope I was clear in my description. If I've left anything out, I'll be
> happy to clarify.
What you likely want to dig into is Query enabled properties
(http://www.sqlalchemy.org/docs/mappers.html?highlight=property%20association#building-query-enabled-properties).
I used this a lot of times to tie what appears to be a relationship but that
is driven by an underlying query. I don't have time right now to whip
up the action property, but if you play with it a bit I'm sure you'll be able
to get there.
Michael
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