Get first element in array and return using Aggregate?

Question

How can I get and return the first element in an array using a Mongo aggregation?

I tried using this code:

db.my_collection.aggregate([
    { $project: {
        resp : { my_field: { $slice: 1 } }
    }}
])

but I get the following error:

uncaught exception: aggregate failed: {
    "errmsg" : "exception: invalid operator '$slice'",
    "code" : 15999,
    "ok" : 0
}

Note that 'my_field' is an array of 4 elements, and I only need to return the first element.

Answer 1

Since 3.2, we can use $arrayElemAt to get the first element in an array

db.my_collection.aggregate([
    { $project: {
        resp : { $arrayElemAt: ['$my_field',0] }
    }}
])