mongodb mapreduce function does not provide skip functionality, is their any solution to this?

Question

Mongodb mapreduce function does not provide any way to skip record from database like find function. It has functionality of query, sort & limit options. But I want to skip some records from the database, and I am not getting any way to it. please provide solutions.

Thanks in advance.

Answer 1

Ideally a well-structured map-reduce query would allow you to skip particular documents in your collection.

Alternatively, as Sergio points out, you can simply not emit particular documents in map(). Using scope to define a global counter variable is one way to restrict emit to a specified range of documents. As an example, to skip the first 20 docs that are sorted by ObjectID (and thus, sorted by insertion time):

db.collection_name.mapReduce(map, reduce, {out: example_output, sort: {id:-1}, scope: "var counter=0")}; 

Map function:

function(){
    counter ++;
    if (counter > 20){
        emit(key, value);
    }
}