Question

How can I iterate over all documents matching each value of a specified key in a MongoDB collection?

E.g. for a collection containing:

{ _id: ObjectId, keyA: 1 },
{ _id: ObjectId, keyA: 2 },
{ _id: ObjectId, keyA: 2 },

…with an index of { keyA: 1 }, how can I run an operation on all documents where keyA:1, then keyA:2, and so on?

Specifically, I want to run a count() of the documents for each keyA value. So for this collection, the equivalent of find({keyA:1}).count(), find({keyA:2}).count(), etc.

UPDATE: whether or not the keys are indexed is irrelevant in terms of how they’re iterated, so edited title and description to make Q/A easier to reference in the future.

Answer 1

A simpler approach to get the grouped count of unique values for keyA would be to use the new Aggregation Framework in MongoDB 2.2:

eg:

db.coll.aggregate(
  { $group : {
     _id: "$keyA",
     count: { $sum : 1 }
  }}
)

… returns a result set where each _id is a unique value for keyA, with the count of how many times that value appears:

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