Get the set of all unique values in array field

Question

Given the following documents:

{ "_id" : ObjectId("585901b7875bab86885cf54f"), "foo" : 24, "bar" : [ 1, 2, 5, 6 ] }
{ "_id" : ObjectId("585901be875bab86885cf550"), "foo" : 42, "bar" : [ 3, 4 ] }

I want to get all the unique values in the bar field, something like:

{"_id": "something", "bar": [1, 2, 3, 4, 5, 6]}

This is what I tried:

db.stuff.aggregate([{
  $group: {
    _id: null, 
    bar: {
      $addToSet: {$each: "$bar"}
    }
  }
}])

But complains that $each is not a recognized operator.

This does work:

db.stuff.aggregate([{
  $group: {
    _id: null, 
    bar: {
      $addToSet: "$bar"
    }
  }
}])

But obviously produces a wrong result:

{ "_id" : null, "bar" : [ [ 3, 4 ], [ 1, 2, 5, 6 ] ] }

---

EDIT

I managed to have the result I want by adding a first $unwind stage:

db.stuff.aggregate([{
  $unwind: { "$bar" },
  $group: {
    _id: null, 
    bar: {
      $addToSet: "$bar"
    }
  }
}])

=> { "_id" : null, "bar" : [ 4, 3, 5, 2, 6, 1 ] }

Is it possible at all to make it in one single pipeline stage?

Answer 1

The distinct() works with array field as well so will beautifully do this.

db.stuff.distinct('bar')

The aggregation framework is overkill for this and will not perform well