The $ arrayElemAt statement in aggregation with Mongo <3.2

Question

Using the aggregation structure in Mongo,

How can I achieve the same result in Mongo <3.2 , as in Mongo 3.2 with the $ arrayElemAt operation ?

Example in Mongo 3.2

Collection

{ "_id" : 1, "name" : "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] }

Inquiry

db.users.aggregate([
   {
     $project:
      {
         name: 1,
         first: { $arrayElemAt: [ "$favorites", 0 ] },
         last: { $arrayElemAt: [ "$favorites", -1 ] }
      }
   }
])

It works great

But I was forced to use Mongo 3.0, so I can’t use this operator, which is perfect for what I want to do

Is there a way to access an array element by index with something like ...

Mongo 3.0

Collection

{ "_id" : 1, "name" : "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] }

Inquiry

db.users.aggregate([
   {
     $project:
      {
         name: 1,
         first: { "$favorites.0" },
         last:  { "$favorites.1" }
      }
   }
])

I tried and it does not work, it gives me an empty array.

However, when they are properties, not indexes, it can retrieve values

Any help would be really appreciated.

thank

Answer 1

$unwind, $group $first $last :

db.collection.aggregate([
    { "$unwind": "$favorites" },
    { "$group": { 
        "_id": "$_id",
        "first": { "$first": "$favorites" },
        "last": { "$last": "$favorites" }
    }}
])

, $unwind

var first = db.collection.find({}, {"favorites": { $slice: 1}})
var last = db.collection.find({}, {"favorites": { $slice: -1}})