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mongodbmongooseaggregation-framework
Hongbo Miao
Hongbo Miao

Reputation: 49774

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

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

Upvotes: 100

Views: 81841

Answers (3)

Michał Szkudlarek
Michał Szkudlarek

Reputation: 1502

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 }

Upvotes: 21

Alex Blex
Alex Blex

Reputation: 37038

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
}},

Upvotes: 193

Xavier Guihot
Xavier Guihot

Reputation: 61666

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" }

Upvotes: 11

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