Is it possible to return a calculated field from a MongoDB query?

Question

In SQL I could do something like

SELECT myNum, (myNum+1) as `increment` FROM myTable

effectively doing arbitrary math and other functions and

Answer 1

The new Aggregation Framework in MongoDB 2.2 allows you to add calculated fields via the $project operator. This isn't quite the same as arbitrary functions because you need to use supported operators, but it does provide a good deal of flexibility.

Here is your example of incrementing _ids into a new myNum field:

MongoDB shell version: 2.2.0-rc0

> db.test.insert({_id:123});

> db.test.insert({_id:456});

> db.test.aggregate(
  { $project : {
      _id : 1,
     'myNum': { $add: [ "$_id", 1]}
  }}
)
{
    "result" : [
        {
            "_id" : 123,
            "myNum" : 124
        },
        {
            "_id" : 456,
            "myNum" : 457
        }
    ],
    "ok" : 1
}