MongoDB: Iterate over collection by key? - mongodb

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
}

Answer 2

I am not sure I get you here but is this what you are looking for:
db.mycollection.find({ keyA: 1 }).count()
Will count all keys with keyA being 1.
If that does not answer the question do think you can be a little more specific?
Do you mean to do an aggregation for all unique key values for keyA?

Answer 3

It may be implemented with multiple queries:
var i=0;
var f=[];
while(i!=db.col.count()){
var k=db.col.findOne({keyA:{$not:{$in:f}}}).keyA;
i+=db.col.find({keyA:k}).count();
f.push(k);
}
The sense of this code is to collect unique values of KeyA field of objects of col collection in array f, which will be result of operation. Unfortunately, for a while doing this operation you should block any operations, which will change col collection.
UPDATE:
All can be done much easier using distinct:
db.col.distinct("KeyA")

Answer 4

Thanks to #Aleksey for pointing me to db.collection.distinct.
Looks like this does it:
db.ships.distinct("keyA").forEach(function(v){
db.ships.find({keyA:v}).count();
});
Of course calling count() within a loop doesn't do much; in my case I was looking for key-values with more than one document, so I did this:
db.ships.distinct("keyA").forEach(function(v){
print(db.ships.find({keyA:v}).count() > 1);
});