Rails - Oct 2007 - Managing a lopsided has_and_belongs_to_many relationship

I''ve got a fixed list of several thousand Things, and a User base that
may grow to who knows how big (we can only hope ,right?)  Users have and
belong to many Things.  I want to display a list of top Things, based on
how many users each has.  Things with 0 users are not displayed.

Two questions.  First, I expect the density of the relatioships to focus
on a small percentage of the things.  So while I may have 10,000 of
them, it might be true that only 100 have at least 1 user.  In doing my
find statements, what would the syntax be to say "I am only interested
in Things where users.size > 0"?  I''m hoping that I
odn''t have to get
them all and simply choose not to display 90% of them, that seems like
unnecessary moving around of data.

Second, my goal is to sort these Things for presentation by users.size()
descending.  Can I push my luck and get that out of the initial find
statement, or do I have to do my own sort?

Thanks!
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Duane Morin wrote:
> I''ve got a fixed list of several thousand Things, and a User base
that
> may grow to who knows how big (we can only hope ,right?)  Users have and
> belong to many Things.  I want to display a list of top Things, based on
> how many users each has.  Things with 0 users are not displayed.
> 
> Two questions.  First, I expect the density of the relatioships to focus
> on a small percentage of the things.  So while I may have 10,000 of
> them, it might be true that only 100 have at least 1 user.  In doing my
> find statements, what would the syntax be to say "I am only interested
> in Things where users.size > 0"?  I''m hoping that I
odn''t have to get
> them all and simply choose not to display 90% of them, that seems like
> unnecessary moving around of data.
> 
> Second, my goal is to sort these Things for presentation by users.size()
> descending.  Can I push my luck and get that out of the initial find
> statement, or do I have to do my own sort?
I''d recommend using a has_many :through if you''re doing stuff
like that.
Here''s an old article I wrote on doing something similar (should be 
updated RSN for 2.0) that should be enough to get you started...

http://blog.hasmanythrough.com/2006/6/12/when-associations-arent-enough-part-2

The counter_cache functionality has been beefed up some since then, and 
you may find that useful too.

happy friday...

--
Josh Susser
http://blog.hasmanythrough.com
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