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
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" }