Question

I have a set of objects in a mongodb database that have measurements embedded. I’m trying to do a map/reduce to get stats about the measurements. Well I ran into some problems with that so I just made it as simple as possible: get the number of measurements for each one.

m = function() {
    emit (mp, { meas:  this.measurements }); 
 };

r = function() {
    return { count: meas.length };
 };

 res = db.meas_points.mapReduce(m, r,
    {query : { measurements : {$exists: true}}},
    {out: { "measurements_stats" }} );

When I run this query I get an error:

Mon Jan 2 16:05:53 SyntaxError: missing : after property id (shell):1

I’m trying to see what this means in the context of my map/reduce but I’m just not seeing it. I lifted the code from the mongodb website ( http://www.mongodb.org/display/DOCS/MapReduce Shell Example 2) and adapted it to my needs. Seems like I’m doing everything right but I keep getting this cryptic error. I’m not using the id field at all — is it possible that I have a malformed record or something?

Answer 1

In this line

return { count: meas.length };

It is not able to resolve meas You need to modify the signature of the reduce function.

var r = function(mp, measValues) {
                      return Array.sum(measValues.length);
                  };

Here measValues passed to this reduce function itself is an array

Note: If mp in the map function is the id/key of your collection meas_points then change that line to this.mp