MongoDB aggregation: Group all records into a single result

Question

In a mongodb aggregation call, how do I use the $group operator to group all documents in the pipeline into a single result?

Let's say I have a collection of records that look like this:

{
    user_id: 234,
    signup_date: 2013-06-27
}

I want to use the aggregate function to query the database for a list of users who signed up in a given date range, and return it as a list. I'd like a result that looks like this:

{
    users: [234, 235, 236]
}

My query looks something like this:

db.users.aggregate([

    { $match: {
        signup_date: {
            $gte: date_begin_variable,
            $lt: date_end_variable
    }},

So far, so good. I now have a subset of records, all of which have a signup date in the desired range. But here's where I get into trouble. Now I want to group ALL those records into a single result containing a list of all the IDs. But I don't know how to do that using $group.

    { $group: {
        _id: null
        users: { $addToSet: user_id }
    }}

])

When I tried _id: null, I got back an empty list of user IDs.

I could just return the desired set of records using find(), and then just process the IDs into a list manually, but doing it using aggregate() seems cleaner. Am I wrong about that? What would be the best way to accomplish what I'm describing, either using aggregate or not?

Answer 1

Try this query:

db.users.aggregate([
  {
    $match: {
      signup_date: {
        $gte: date_begin_variable,
        $lt: date_end_variable
      }
    }
  },
  {
    $group: {
      _id: null,
      users: {
        $addToSet: "$user_id"
      }
    }
  }
])