mongodb: return an array of document ids

Question

Is it possible to query mongodb to return array of matching document id values, without the related keys?

Please consider following 'parent' data structur:

{
  "_id": ObjectId("52448e4697fb2b775cb5c3a7"),
  "name": "Peter",
  "children": [
    {
      "name": "joe"
    }
  ]
},
{
  "_id": ObjectId("52448e4697fb2b775cb5c3b6"),
  "name": "Marry",
  "children": [
    {
      "name": "joe"
    }
  ]
}

I would to query for an array of parent _ids whose children have the name "joe"

For provided sample data, I would like the following output returned from mongo:

[ObjectId("52448e4697fb2b775cb5c3a7"), ObjectId("52448e4697fb2b775cb5c3b6")]

I know that I can query for an output like this, which also contains the keys

[{"_id": ObjectId("52448e4697fb2b775cb5c3a7")}, {"_id": ObjectId("52448e4697fb2b775cb5c3b6")}]

However I need to push above array to another document with an update operation like this:

db.statistic.update({"date": today}, {$push: {"children": [ObjectId("52448e4697fb2b775cb5c3a7"), ObjectId("52448e4697fb2b775cb5c3b6")]}}, true, false)

I would like to avoid sorting out the document structure, in case it is possible to just return an array containing the appropriate values using mongo

Answer 1

It should be possible by

db.coll.distinct("_id", {"children.name": "joe"})