How to count occurrences in nested document in mongodb?

Question

In the situation where there is an array nested within an array, how would I count the number of a specific value? For example, I want to count the number of \"answers\" in the

Answer 1

Here is one way to skin your cat using the aggregate framework. Once you learn it you can do a lot of good stuff with you data.

db.so.aggregate([{$unwind:"$questions"}, {$unwind:"$questions.answers"}, {$group:{_id:"$questions.answers.answer", fruit_count:{$sum:1}}}])

Gives me this:

{
    "result" : [
        {
            "_id" : "banana",
            "fruit_count" : 1
        },
        {
            "_id" : "apple",
            "fruit_count" : 2
        }
    ],
    "ok" : 1

To be doubly sure, I added this doc:

db.so.insert({
  "_id" : ObjectId("52c1d909fc7fc68ffffd999a75"),
  "name" : "Some survey",
  "questions" : [
    {
      "_id" : ObjectId("52c1e250fc7fc68ffffd999a75"),
      "answers" :
      [
        {
          "userId" : "some GUIDs",
          "answer" : "orange"
        },
        {
          "userId" : "some GUID",
          "answer" : "orange"
        },
        {
          "userId" : "some GUID",
          "answer" : "banana"
        }
      ],
      "questionText" : "blah blah blah...",
      "questionType" : "multiple choice"
    }
]
})

My query now gives me:

{
    "result" : [
        {
            "_id" : "orange",
            "fruit_count" : 2
        },
        {
            "_id" : "banana",
            "fruit_count" : 2
        },
        {
            "_id" : "apple",
            "fruit_count" : 2
        }
    ],
    "ok" : 1
}