Count subdocuments for each document

Question

I'm trying to just count subdocuments in Mongo for each document.

It's easy to get documents in a collection with db.users.find().length(). I want to do something similar like db.users.projects.find().length(). How can I do this?

Edit:

The result I want is to know how many projects each user has... So something like:

{_id: 123, projects: 4}

Example user document:

{
   _id:{$oid: 123},
   username: johnsmith,
   projects: [{$oid: 456}, {$oid: 789}]
}

Answer 1

Per @n9code, you will need the Aggregation Framework. However, you can very easily count the sub-documents with $size:

db.users.aggregate([{
  $project: {
    _id: '$_id',
    totalProjects: { $size: "$projects" }
  }
}]);

Which should return something like this:

{ "_id" : ObjectID(...), "totalProjects" : 89 }, ...

$size will return the length of the projects array for each document and $project changes the format of the documents to include totalProjects as the size of the projects array.

Answer 2

It seems like you may have multiple username docs in your design, e.g.

{ username: "buzz", projects: [ list of 2 things ] }
{ username: "dan". projects: [ list of 3 things ] }
{ username: "buzz", projects: [ list of 4 things ] }

To arrive at the "grant total" of projects for each username try this:

c = db.foo.aggregate([
{$project: {
    "username": 1,
    "n": {$size: "$projects"}
}
}
, 
{$group: {
    "_id": "$username",
    "alln": {$sum: "$n"}
}
}
]);

to yield

{ "_id" : "buzz", "alln" : 6 }
{ "_id" : "dan", "alln" : 3 }