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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: Howto drop rows with reduce?
Date Wed, 29 Apr 2009 05:31:33 GMT
Jurg van Vliet wrote:
> hi people,
>
> (this is my first 'sound' on this list, so hi again. i am enjoying 
> couchdb, especially when seeing how many people are having fun doing 
> 'something different'. so, thank you for showing me what you are doing.)
>
> for our project we am using couchdb as the general datastore. we store 
> everything in a document in couchdb, and we distiguish type explicitly 
> (with a field in the document.) suppose we have articles and files and 
> comments.
>
> both articles and files have (nested) comments. a comment stores its 
> path, and this way models its ancestry. there is no reference from 
> either article or file to comments, as this is not necessary.
>
> (getting this to work with couchdb already forced us to learn (and 
> accept) couchdb for what it is :) difficult, but fun!)
>
> now, suppose we want to list articles ordered by the number of 
> comments. at first glance this is easy, and the solution is similar to 
> the tag solutions you find by googling. but we want a view that gives 
> us only the articles, we don't want the files. (we want separate views 
> for files, and other types of documents containing comments.)
>
> the solution we thought of was 'a bit of a hack', but it should work. 
> the map code for the document type article looks like this
>
> function(doc) {
> if( doc.type == 'Article') {
> emit( doc._id, 0);
> } else if( doc.type == 'Comment') {
> emit( doc.path[0], 1);
> } else {
> emit( doc._id, "a");
> }
> }
>
Is there a reason you have this last emit statement? There's nothing 
forcing you to emit anything. So you could just delete the last if 
clause and then those docs won't be in your reduce. In the only way to 
drop a reduce row is to not emit it.

Paul Davis
> here you see that we emit everything, but we emit a non-number value 
> when the type is not what we want. a simple reduce would drop the rows 
> that have values that are not-a-number leaving us with exactly what we 
> want. the reduce could look like this:
>
> function( keys,values,rereduce) {
> var total = sum( values);
>
> if( !isNaN( total)) {
> return total;
> }
> }
>
> but this gives an error about a 'bad_cause', at least in futon. adding 
> something like 'return null' doesn't help very much, because it 
> doesn't exclude what we don't want to see.
>
> my questions are
> 1. how can we exclude things from the result of the view with reduce, 
> in other words 'how to drop rows with reduce'?
> 2. how else can you attack this problem?
>
> thanks,
> jurg.
>
> a little of bit of context: we are creating a rails application 
> following basic_model and couchrest. everyone is still struggling with 
> understanding couchdb, including us. so we decided to stick with this 
> for a while, and patch little things for what we need in our 
> application. we choose to design the documents as close as possible to 
> the rails models. as a result we implemented views on the level of a 
> model, because that is the way we look at our problem. we have certain 
> views for articles, for files, for users, etc. perhaps this direction 
> of implementation doesn't get us to where we want to go. so (almost) 
> any suggestion is welcome:) we still want to have multiple types of 
> information in one couchdb database, though.


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