mongoose - Project first item in an array to new field (MongoDB aggregation)

Question

I am using Mongoose aggregation (MongoDB version 3.2).

I have a field users which is an array. I want to $project first item in this array to a new field user.

I tried

  { $project: {
    user: '$users[0]',
    otherField: 1
  }},

  { $project: {
    user: '$users.0',
    otherField: 1
  }},

  { $project: {
    user: ｛ $first: '$users'},
    otherField: 1
  }},

But neither works.

How can I do it correctly? Thanks

Answer 1

Update:

Starting from v4.4 there is a dedicated operator $first:

{ $project: {
    user: { $first: "$users" },
    otherField: 1
}},

It's a syntax sugar to the

Original answer:

You can use arrayElemAt:

{ $project: {
    user: { $arrayElemAt: [ "$users", 0 ] },
    otherField: 1
}},