example couchdb map/reduce

gistfile1.js

/*
  So with documents as such:
  {
     one_id : 1,
     another_id : 22,
     created_at : "2011-05-26 23:22:11",
     a_name : "Lisa"
  }

  We want to get a count of the unique 'a_name' values possibly 
  filtering by the 'created_at' or the id fields.
*/

//Map:
function (doc) {
    emit([doc.created_at, doc.one_id, doc.another_id], doc.a_name);
}

//Reduce:
function (key, values, rereduce) {
    if (rereduce) {
        //rereduce is true:
        //key is null
        //values is an array of the results we have returned from the reduce

        //now aggregate the original reduce results
        //which are in documents of { <name>: <count> ... }
        //structure
        var results = { };
        for(var i = 0; i < values.length; i++) {
            for(var name in values[i]) {
                if (results[name]) results[name] += values[i][name];
                else results[name] = values[i][name];
            }
        }
        return results;
    }

    //rereduce is false:
    //key will be an array of the map's emitted keys
    //values will be an array of the map's emitted values in the same
    //order as the keys

    //group and count the docs by the name
    //which is the value the map function emitted
    var results = { };
    for(var i = 0; i < values.length; i++) {
        var name = values[i];
        if (results[name]) results[name]++;
        else results[name] = 1;
    }
    return results;
}

/*
  example results:
  { John: 4, Jane: 6 }
*/