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

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
}},

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 } 

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" }