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From Josh Steiner <vitrio...@gmail.com>
Subject How to match a superset key to a subset using group_level (sub selects in couchdb?)
Date Tue, 08 Mar 2011 19:54:34 GMT
How to do sub selects in couchdb, or, how to match a superset key to a
subset using group_level

I'm new to both CouchDB and writing Map/Reduce views and I have a
pretty complex question that hopefully has a not too complex answer to
someone who's not just learning map/reduce and couchdb for the first
time.

I am working on a system that serves a json manifest to a client to
configure itself with content that updates daily. On first run the
clients register themselves with a few descriptive tags (say: screen
size, OS, location), and the server returns back a group_id. The
client uses that id to request its manifest every day. On the backend
we arbitrarily group clients together that share certain tags to cut
down on the number of unique manifests we need to store/serve.

Our sales/admin person has a webapp where he can setup audiences to
target specific content at specific groups. An audience can overlap
multiple groups. The trick is, when the client reports in to get a
fresh manifest we need to figure out which audience is the best fit to
that client's group. The best matching audience will be the first
audience who's tags are a subset of the submitted groups tags, e.g.:

audience1: tagA, tagB, tagC, tagD
audience2: tagA, tagC

group1: tagA, tagB, tagC
This group should match audience2, not audience1.

If we were using an audiences tags to find the best group match (in
other words, if group.tags were a subset of audience.tags) I could
build a really effecient index like so:

[tagA, tagB, tagC], group1._id
[tagA, tagC, tagB], group1._id
[tagB, tagA, tagC], group1._id
[tagB, tagC, tagA], group1._id
[tagC, tagA, tagB], group1._id
[tagC, tagB, tagA], group1._id
and use group_level=2 with key=[tagA, tagC] to match audience2 against
the second line in the index. The problem is, I can't figure out how
to do this going the other direction: matching a group.tags against an
index of audience.tags, where the tags we know at query time
(group.tags) are a superset of the tags we are trying to match against
(audience.tags)

I've got a firm grasp on simple m/r views, but I keep hiting dead ends
on this one. Every solution I come to involves doing some sort of sub
select in my view function, which doesn't work in couchdb views... any
ideas on how I can attack a problem like this?

Hopefully this decription makes some sense.  I'm also open to hearing
"you are approaching this totally wrong" :)

Thanks,

-Josh

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