mongodb - How to lookup into same collection in an array of id?

Question

I have a user schema with

var userSchema = new mongoose.Schema(
{
  username: {
    type: String,
    required: true,
  },
  follows : [{
    type : Schema.Types.ObjectId, 
    ref : "User"
  }],
  followers : [{
    type : Schema.Types.ObjectId, 
    ref : "User"
  }],
});

I have a userId, and then I want to fetch all followers from that useriD. I am new to mongodb.

So a user have a field of followers[], it will containen the userid of all followers. So i want to fetch all fields of the followers.

question from:https://stackoverflow.com/questions/65869425/how-to-lookup-into-same-collection-in-an-array-of-id

Answer 1

• $match your user id condition
• $lookup to join, pass same collection name in from

User.aggregate([
  { $match: { _id: 1 } },
  {
    $lookup: {
      from: "user", // update to your actual collection name
      localField: "followers",
      foreignField: "_id",
      as: "followers"
    }
  }
])

Playground