Counting field names (not values) in mongo database - mongodb

Question

Is there a way to count field names in mongodb? I have a mongo database of documents with other embedded documents within them. Here is an example of what the data might look like.
{
"incident": "osint181",
"summary":"Something happened",
"actor": {
"internal": {
"motive": [
"Financial"
],
"notes": "",
"role": [
"Malicious"
],
"variety": [
"Cashier"
]
}
}
}
Another document might look like this:
{
"incident": "osint182",
"summary":"Something happened",
"actor": {
"external": {
"motive": [
"Financial"
],
"notes": "",
"role": [
"Malicious"
],
"variety": [
"Hacker"
]
}
}
}
As you can see, the actor has changed from internal to external in the second document. What I would like to be able to do is count the number of incidents for each type of actor. My first attempt looked like this:
db.public.aggregate( { $group : { _id : "$actor", count : { $sum : 1 }}} );
But that gave me the entire subdocument and the count reflected how many documents were exactly the same. Rather I was hoping to get a count for internal and a count for external, etc. Is there an elegant way to do that? If not elegant, can someone give me a dirty way of doing that?

Answer 1

Best option for this kind of problem is using map-reduce of mongoDB , it will allow you to iterate through all the keys of the mongoDB document and easily you can add your complex logic . Check out map reduce examples here : http://docs.mongodb.org/manual/applications/map-reduce/

Answer 2

This was the answer I came up with based on the hint from Devesh. I create a map function that looks at the value of actor and checks if the document is an empty JSON object using the isEmptyObject function that I defined. Then I used mapReduce to go through the collection and check if the action field is empty. If the object is not empty then rather than returning the value of the key, I return the key itself which will be named internal, or external, or whatever.
The magic here was the scope call in mapReduce which makes it so that my isEmptyObject is in scope for mapReduce. The results are written to a collection which I named temp. After gathering the information I want from the temp collection, I drop it.
var isEmptyObject = function(obj) {
for (var name in obj) {
return false;
}
return true;
};
var mapFunction = function() {
if (isEmptyObject(this.action)) {
emit("Unknown",1); }
else {
for (var key in this.actor) { emit(key,1); } } };
var reduceFunction = function(inKeys,counter) {
return Array.sum(counter); };
db.public.mapReduce(mapFunction, reduceFunction, {out:"temp", scope:{isEmptyObject:isEmptyObject}} );
foo = db.temp.aggregate(
{ $sort : { value : -1 }});
db.temp.drop();
printjson(foo)