Find all duplicate documents in a MongoDB collection by a key field

Question

Suppose I have a collection with some set of documents. something like this.

{ \"_id\" : ObjectId(\"4f127fa55e7242718200002d\"), \"id\":1, \"name\" : \"foo\"         


        

Answer 1

Note: this solution is the easiest to understand, but not the best.

You can use mapReduce to find out how many times a document contains a certain field:

var map = function(){
   if(this.name) {
        emit(this.name, 1);
   }
}

var reduce = function(key, values){
    return Array.sum(values);
}

var res = db.collection.mapReduce(map, reduce, {out:{ inline : 1}});
db[res.result].find({value: {$gt: 1}}).sort({value: -1});