couchdb map function and reduce function

Question

I am trying to work out the map functions and reduce functions for the following case:

documents:

{
 "_id" : "1_A",
 "key1": 10,
 "key2": 10
}

{
 "_id" : "2_A",
 "key1": 2,
 "key2": 3
}
{
 "_id" : "1_B",
 "key1": 20,
 "key2": 20
}

{
 "_id" : "2_B",
 "key1": 1,
 "key2": 0
}

etc.

expecting to have: the sum of each key (key1, key2... keyn) in all documents with id ends with "_A", "_B" ... "_Z" individually.

in this case, expecting to have sumA: "key1":12, "key2":13. and sumB: "key1":21, "key2":20;

I have been working on the map function(doc), it seems like to me that the it is only to deal with one document which specified as the "doc" value at a time.

Is there anyway to achieve the result as expected? is there anyway like SQL join???

was my answer helpful to you? or do you need more information i.e. I was not giving enough details? — thriqon

thriqon thriqon · Accepted Answer · 2013-07-27T17:23:36

yes, this is possible! You will have to emit a compound key here, like this: ["A", 1] for the value of key in a document with an id ending with A.

example for map:

function (doc) {
    for (var k in doc) {
        if (k !== '_id' && k !== 'rev') {
            emit([doc._id.substr(doc._id.length - 1), k.substr(k.length - 1)], 
                doc[k]);
        }
    }
}

For reduce you want to use the builtin function _sum for performance.

You can then query these sums using group=true as URL parameter, possibly restricting the values to only those with suffix A by using startkey and endkey (e.g. ["A"] and ["A", {}]).

In this case the result would be:

{"rows":[
    {"key":["A","1"],"value":12},
    {"key":["A","2"],"value":13},
    {"key":["B","1"],"value":21},
    {"key":["B","2"],"value":20}
]}

couchdb map function and reduce function

1 Answers