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 document below. query should return that there are 2 apples and 1 banana.

{
  "_id" : ObjectId("52c1d909fc7fc68ddd999a73"),
  "name" : "Some survey",
  "questions" : [
    {
      "_id" : ObjectId("52c1e250fc7fc68ddd999a75"),
      "answers" :
      [
        {
          "userId" : "some GUIDs",
          "answer" : "apple"
        },
        {
          "userId" : "some GUID",
          "answer" : "apple"
        },
        {
          "userId" : "some GUID",
          "answer" : "banana"
        }
      ],
      "questionText" : "blah blah blah...",
      "questionType" : "multiple choice"
    }
]

}

Answer 1

There are a few ways to approach this depending on how much data you need to process. You could use the Aggregation Framework in MongoDB 2.2+, or possibly Map/Reduce. See Aggregation Commands Comparison for a summary of the features and limitations.

Here's an example using the Aggregation Framework:

db.fruit.aggregate(
    // Limit matching documents (can take advantage of index)
    { $match: {
        "_id" : ObjectId("52c1d909fc7fc68ddd999a73")
    }},

    // Unpack the question & answer arrays
    { $unwind: "$questions" },
    { $unwind: "$questions.answers" },

    // Group by the answer values
    { $group: {
        _id: "$questions.answers.answer",
        count: { $sum: 1 }
    }}
)

For your sample document this returns:

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

Answer 2

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("52c1d909fc7fc68ddd999a75"),
  "name" : "Some survey",
  "questions" : [
    {
      "_id" : ObjectId("52c1e250fc7fc68ddd999a75"),
      "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
}