How to get all the students of a department in Mongoose MongoDb

Question

I am an amateur in MongoDB, trying to build an educational database.

I have two document models: 1) Department 2) Student

Each Student document has ref: Department with its _id.

So, If I have the Department _id, how can I find all the students it has?

const department = mongoose.schema({
  name: { type: String } 
})
const student = mongoose.schema({
  name: { type: String },
  department: { mongoose.Schema.Types.ObjectId, ref: Department}
})

Answer 1

You only need to use $lookup like this:

Using this $lookup you are telling mongo "give me all documents from student collection where its department field is like _id field in department collection and create an array called departmens with these values.

yourDepartmentModel.aggregate([
  {
    "$lookup": {
      "from": "student",
      "localField": "_id",
      "foreignField": "department",
      "as": "departments"
    }
  }
])

Example here