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

Possible duplicate of [Get n-th element of an array in MongoDB](http://stackoverflow.com/questions/7223273/get-n-th-element-of-an-array-in-mongodb) — styvane, Aug 28 '16 at 23:21

Alex Blex · Accepted Answer · 2021-06-29T07:54:36.253

179

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
}},

edited Jun 29 '21 at 07:54

answered Aug 28 '16 at 23:16

Alex Blex

34,704
7
48
75

12

how can we directly project array element's key? like.. [{ name: 'foo' }] , I want to project name directly. – Sachin Bhandari Aug 12 '18 at 13:29
14

@SachinBhandari { $arrayElemAt: [ "$users.key.anotherNestedKey", 0 ] } – Marian Gibala May 22 '19 at 16:55
It is returning one element - but still returning an array :( – Saw Feb 21 '23 at 14:50
@Saw https://www.mongodb.com/docs/manual/reference/operator/aggregation/first-array-element/#behavior says it should be the item itself. – Alex Blex Feb 21 '23 at 15:50
Had to do user: { $first: { $first: "$users" } } - although I didn't have array of arrays – Saw Feb 21 '23 at 18:06

Michał Szkudlarek · Answer 2 · 2021-07-15T06:57:07.527

19

If it is an array of objects and you want to use just single object field, ie:

{
 "users": [
    {name: "John", surname: "Smith"},
    {name: "Elon", surname: "Gates"}
   ]
}

you can use:

{ $project: { user: { $first: "$users.name" } }

Edit (exclude case - after comment from @haytham)

In order to exclude a single field from a nested document in array you have to do 2 projections:

 { $project: { user: { $first: "$users" } }

Which return whole first object, and then exclude field you do not want, ie:

{ $project: { "user.name" : 0 }

edited Jul 15 '21 at 06:57

answered Mar 05 '21 at 11:51

Michał Szkudlarek

1,443
1
21
35

how can we exclude the match? I mean how to pass the 0 after that? { $project: { user: { $arrayElemAt: [ "$users", 0 ] }, otherField: 1 }}, or { $project: { user: { $first: "$users.name" } } – haytham Jul 14 '21 at 20:35
@haytham - added example to the answer. I think this cannot be done in a single projection ($first + exclude field from the result of first). – Michał Szkudlarek Jul 15 '21 at 06:58

score 11 · Answer 3 · answered Jan 24 '20 at 21:05

Starting Mongo 4.4, the aggregation operator $first can be used to access the first element of an array:

// { "users": ["Jean", "Paul", "Jack"] }
// { "users": ["Claude"] }
db.collection.aggregate([
  { $project: { user: { $first: "$users" } } }
])
// { "user" : "Jean" }
// { "user" : "Claude" }

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

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