Question

Let’s say I have 4 documents in the database:

{ name: 'alex' }
{ name: 'jen' }
{ name: 'alex' }
{ name: 'john'}

In MongoDB Shell, I want to know if 2 or more documents share the same ‘name’.

Basically, if all the names are different, return true.
If 2 or people have the same name, return false.

Answer 1

Try this, uses a quick map-reduce query to find the number of documents that have the same name, and returns true if they’re all different:

function allDifferent() {
    var m = function() { emit(this.name, 1); }
    var r = function(key, emits) {
        var n = 0; emits.forEach(function(v) { n += v; }); return n;
    }
    var result = db.mycol.mapReduce(m, r, { out: "namecounts" });
    var allDifferent= (db.namecounts.count( { value: { $gt: 1 } } ) == 0)
    db.namecounts.drop();
    return allDifferent;
}