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

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("52c1d909fc7fc68ffffd999a73")
    }},

    // 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
}