Count unique values within model's Array in MongoDB

Question

What is the best way to just COUNT unique values within many model\'s array? Let\'s say my Model is like this:

{
  myKey: [
    \"idnumber1000\",
    \"idnumber1         


        

Answer 1

You can do this with a simple aggregate pipeline:

MyModel.aggregate([
    // Project just the myKey field as that's all that's needed
    {$project: {_id: 0, myKey: 1}},
    // Duplicate each doc, once per myKey element
    {$unwind: '$myKey'},
    // Group on myKey and get a count
    {$group: {_id: '$myKey', count: {$sum: 1}}}
  ],
  function(err, results) {...}
);