Question

I am trying to perform a map reduce directly into Mongo to get the top tags but I get this error and I am not sure what it means and what do with it. Any guidance?

> reduce= function(previous, current) {
... var count = 0;
... 
...     for (index in current) {
...         count += current[index];
...     }
... 
...     return count;
... }
function (previous, current) {
    var count = 0;
    for (index in current) {
        count += current[index];
    }
    return count;
}
> reduce
function (previous, current) {
    var count = 0;
    for (index in current) {
        count += current[index];
    }
    return count;
}
> map = function() {
... if (!this.stores) {
... return;
...     }
... for (index in this.stores) {
... emit(this.stores[index], 1);
...  }
... }
function () {
    if (!this.stores) {
        return;
    }
    for (index in this.stores) {
        emit(this.stores[index], 1);
    }
}
> map
function () {
    if (!this.stores) {
        return;
    }
    for (index in this.stores) {
        emit(this.stores[index], 1);
    }
}
> result = db.runCommand({
... "mapreduce" : "users",
... "map" : map,
... "reduce" : reduce,
... "out" : "tags"})
{
    "assertion" : "_id cannot be an array",
    "assertionCode" : 10099,
    "errmsg" : "db assertion failure",
    "ok" : 0
}

Answer 1

"assertion" : "_id cannot be an array" 

might mean that your this.stores[index] is an array cause the MapReduce result has _id as your Map key