Mongodb $lookup Not working with _id

Question

wend try with this query, return the lookup is empty

db.getCollection('tests').aggregate([
    {$match: {typet:'Req'}},
    {$project: {incharge:1}},
    {$lookup:{
            from: "users",
            localField: "incharge", //this is the _id user from tests
            foreignField: "_id", //this is the _id from users
            as: "user"
    }}
])

return json

  [
    {
        "_id": "57565d2e45bd27b012fc4db9",
        "incharge": "549e0bb67371ecc804ad23ef",
        "user": []
    },
    {
        "_id": "57565d2045bd27b012fc4cbb",
        "incharge": "549e0bb67371ecc804ad21ef",
        "user": []
    },
    {
        "_id": "57565d2245bd27b012fc4cc7",
        "incharge": "549e0bb67371ecc804ad24ef",
        "user": []
    }
]

i try with this post but nothing happend MongoDB aggregation project string to ObjectId and with this MongoDB $lookup with _id as a foreignField in PHP

UPDATE

this is the Document "users"

    {
        "_id" : ObjectId("549e0bb67371ecc804ad24ef"),
        "displayname" : "Jhon S."
    },
    {
        "_id" : ObjectId("549e0bb67371ecc804ad21ef"),
        "displayname" : "George F."
    },
    {
        "_id" : ObjectId("549e0bb67371ecc804ad23ef"),
        "displayname" : "Franc D."
    }

Answer 1

I finaly found the solution, is a problem with my Schema in mongoose with the ObjectId

I change this

var Schema = new Schema({
    name: { type: String, required: true},
    incharge: { type: String, required: true},
});

with this

var Schema = new Schema({
    name: { type: String, required: true},
    incharge: { type: mongoose.Schema.ObjectId, required: true},
});

and is working