$limit number of documents in $lookup - arrays

Question

I encounter a problem with this query that returns this result:
{
"_id" : ObjectId("5bd22f28f77cfb1f6ce503ca"),
"search" : "flarize",
"name" : "flarize",
"color" : 0,
"profil" : "",
"banner" : "",
"desc" : "",
"date" : 1540501286109,
"friend" : [
[
{
"_id" : ObjectId("5bd22f28f77cfb1f6ce503ca"),
"search" : "flarize",
"name" : "flarize",
"email" : "flarize.b73#gmail.com",
"password" : "$2a$10$eYeOtEkEUyD7TFkjKvhZOuSSpvBolkL17TrPHuoHhOT8JrsQR0UKW",
"color" : 0,
"profil" : "",
"banner" : "",
"desc" : "",
"date" : 1540501286109,
"friend" : [
{
"id" : ObjectId("5bd22f28f77cfb1f6ce503ca"),
"date" : 1540572026419
},
{
"id" : ObjectId("5bd19a92da24674fdabd26b6"),
"date" : 1540572026419
}
],
"groupes" : [ ]
}
]
]
}
But it's not really what I want, I'd like these fields to be hidden: friend.password, friend.email and the result of friend.groups and friend.friend is limité to 10. I don't know how i can solve this problem. My request :
db.users.aggregate(
{$match:
{search:"flarize"}
},
{$lookup:
{from:"users",
localField:"friend.id",
foreignField:"_id",
as:"friend"
}},
{$project:
{ search: 1,
name: 1,
profil: 1,
banner: 1,
color: 1,
date: 1,
desc: 1,
friend: [{$slice:["$friend", 0, 10]}]
}
}).pretty();
One document of users:
{
"_id" : ObjectId("5bd22f28f77cfb1f6ce503ca"),
"search" : "flarize",
"name" : "flarize",
"email" : "theo.ba73#gmail.com",
"password" : "$2a$10$eYeOtEkEUyD7TFkjKvhZOuSSpvBolkL17TrPHuoHhOT8JrsQR0UKW",
"color" : 0,
"profil" : "",
"banner" : "",
"desc" : "",
"date" : 1540501286109,
"friend" : [
{
"id" : ObjectId("5bd22f28f77cfb1f6ce503ca"),
"date" : 1540572026419
},
{
"id" : ObjectId("5bd19a92da24674fdabd26b6"),
"date" : 1540572026419
}
],
"groupes" : [ ]
}
Thank you for helping me.

Answer 1

You can try below aggregation
db.users.aggregate([
{ $match: { search: "flarize" } },
{ $lookup: {
from: "users",
let: { friendId: "$friend.id" },
pipeline: [
{ $match: { $expr: { $in: ["$_id", "$$friendId"] }}},
{ $limit: 10 },
{ $project: { email: 0, password: 0 }}
],
as: "friend"
}}
])