Is it possible that mongodb's $project return an array?

Question

Is it possible for MongoDb's $project aggregation operator to restructure the document to an array ?

Here is what I did so far:

var pipeline = [];
var project = {
    $project : {
        x: "$_id", 
        y: "$y" ,
        _id : 0
    }
};
pipeline.push(project);
model.aggregate( pipeline, callback);

This gives me output of form:

[
  { 
   x: '...',
   y: '...'
  }
 ....
]

I would like to have:

[
   ['..','..']
   ....
]

I can easily restructure the output by iterating it, but really curious to know if aggregate itself could return array instead of object.

Answer 1

You could try with the $push operator.

For example, if you had documents like:

{ _id: <something>, y: 5 } 

In the mongo shell, if you type

db.model.aggregate( [ { $group: { _id: null, newArrayField: { $push: {  x: "$_id", y: "$y"  } } } } ] )

You would get:

{
    "result" : [
        {
            "_id" : null,
            "newArrayField" : [
                {
                    "x" : ObjectId("5265dd479eb4b1d4289cf222"),
                    "y" : 5
                }
            ]
        }
    ],
    "ok" : 1
}

For more information on $push operator, see http://docs.mongodb.org/manual/reference/operator/aggregation/push/