Populate specific fields in $lookup

Question

I am using aggregate to group and populate the result like below:

 { 
   \"$group\": {
     \"_id\": \"$userId\",
     \"projectId\": { \"$push\": \"$projectId\         


        

Answer 1

You can use below aggregation with mongodb 3.6 and above

With the newer $lookup syntax you can use $projection inside the $lookup pipeline

db.collection.aggregate([
  { "$group": {
    "_id": "$userId",
    "projectId": { "$push": "$projectId" }
  }},
  { "$lookup": {
    "from": "users",
    "let": { "userId": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$_id", "$$userId" ] }}},
      { "$project": { "firstName": 1 }}
    ],
    "as": "user"
  }},
  { "$unwind": "$user" },
  { "$lookup": {
    "from": "projects",
    "let": { "projectId": "$projectId" },
    "pipeline": [
      { "$match": { "$expr": { "$in": [ "$_id", "$$projectId" ] }}},
      { "$project": { "projectName": 1 }}
    ],
    "as": "projects"
  }}
])