Question

I have created the following Map Reduce and came across something curious. I’m counting in 2 different ways the number of documents per date and coming up with different values. Here are my functions:

map : function Map() {
    emit(
        this.cDate,//Holds a date value
        {
        count: 1, 
        }
    );
}

reduce : function Reduce(key, values) {
    var reduced = {count:0,count1:0};
    values.forEach(function(val) {
        reduced.count += val.count; 
        reduced.count1++;
    });

return reduced;
}

finalize : function Finalize(key, reduced) {
    return reduced;
}

query : { "cDate" : { "$gte" : ISODate("2012-11-20T00:00:00Z") } }

out : { inline : 1 }

So Basically what is strange is that at the end “count” and “count1” are returning different values. “count” has the correct value, that is, the number of documents for that date while “count1” has a much lower value. Can anyone explain (I’m new to MongoDB so use simple terms 🙂
Thanks.

Answer 1

Two problems (which are really the same problem):

• Your emit format must be the same as your result returned in the emit function.

• Your reduce must be prepared to be called more than once for the same key (i.e. if you reduce five values for a key and then reduce three values for a key, the reduce function may be called again to reduce the result of two previous reduce operations.

Your example just demonstrates what happens if you assume that you will always be reducing the result “1” rather than the actual previously emitted or reduced result.

Reference: http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-ReduceFunction