I’m not sure that there is a good way to do with with the facilities CouchDB provides, but I’d like to somehow extract the relative complement of the sets of two different document types over a particular key.
For example, let’s say that I have documents representing users and posts, both of which have a (unique) username field. There’s a validation in place ensuring that a user document exists for the username in every post, but there may be any number post documents with a given username, include none. It’s trivial to create a view which counts the number of posts per username. The view can even include zero-counts by emitting zero post-counts for the user documents in the view map function. What I want to do though is retrieve just the list of users who have zero associated posts.
It’s possible to build the view I described above and filter client-side for zero-value results, but in my actual situation the number of results could be very, very large, and the interesting results a relatively small proportion of the total. Is there a way to do this sever-side and retrieve back just the interesting results?
I would write a map function to iterate through the documents and emit the users (or just usersnames) with 0 posts.
Then I would write a list function to iterate through the map function results and format them however you want (JSON, csv, etc).
(I would NOT use a reduce function to format the results, even if a reduce function appears to work OK in development. That is just my own experience from lessons learned the hard way.)