exclude fields in $lookup aggregation - mongodb

Question

I am querying between 3 collections I want to exclude _id everywhere in output
My output is:
{
"_id" : ObjectId("5b6aed5f9bcdb5d4ae64aef5"),
"userID" : "1",
"skills" : [
{
"_id" : ObjectId("5b766b5f1365a4940bb6050f"),
"skillID" : "javaid",
"skillname" : "जावा",
"languageID" : "hindiid"
},
{
"_id" : ObjectId("5b766b8c1365a4940bb60535"),
"skillID" : "pythonid",
"skillname" : "पायथन",
"languageID" : "hindiid"
}
],
"gender" : {
"_id" : ObjectId("5b7687cd2a2329043e2383d5"),
"genderID" : "femaleid",
"gendername" : "महिला",
"languageID" : "hindiid"
}
}
Query:
db.User.aggregate([
{ "$match": { "userID":"1" }},
{ "$lookup":{
"from": "Skill",
"pipeline": [
{ "$match": { "languageID": "hindiid", "skillID": { "$in": [ "javaid","pythonid" ] }}},
],
"as": "skills"
}},
{ "$lookup": {
"from": "Gender",
"pipeline": [
{ "$match": { "languageID": "hindiid", "genderID" : "femaleid" }},
],
"as": "gender"
}},
{ "$unwind": { "path": "$gender", "preserveNullAndEmptyArrays": true }},
{ "$project": { "userID": 1, "skills": 1, "gender": 1 }}
])
In output for every object has _id.Example for skill list every object has _id i want exclude _id field every where. How I can exclude?

Answer 1

In mongodb 3.6 you can use projection ($project) inside $lookup pipeline... Something like this
db.User.aggregate([
{ "$match": { "userID":"1" }},
{ "$lookup":{
"from": "Skill",
"pipeline": [
{ "$match": { "languageID": "hindiid", "skillID": { "$in": [ "javaid","pythonid" ] }}},
{ "$project": { "_id": 0 }}
],
"as": "skills"
}}
])