In Mongodb , how do I check if all the documents are unique for a value?

Question

Let\'s say I have 4 documents in the database:

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

In MongoDB Shell,

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;
}