Mongoose: How to extract arrays in subdocuments to a single array in the parent document

Question

An Express server using Mongoose returns the following JSON from the database in one of its routes:

{
  "gradeLevel": 12,
  "classes": [
    {
      "className": "A",
      "students": [
        "student1", "student2","student3"
      ]
    },
    {
      "className": "B",
      "students": [
        "student4", "student5","student6"
      ]
    },
    {
      "className": "C",
      "students": [
        "student7", "student8","student9"
      ]
    }
  ]
}

In the client, I'd like to be able to for example efficiently check if a student is in any of the class subdocuments in the parent document. The following are the solutions I've conceived, but am not sure which is best practice:

1. When a student is pushed to class subdocument, push the student to the parent's students field as well.
2. Add a new field in the model's toJSON transformation to compile all the students.
3. Use vanilla Javascript in the client to extract all students

Answer 1

I was at this problem too. But to tackle this you need a separate collection called allStudents for example where the schema of it is as follows { nameOfStudent: 'String' className: 'String' }

then aggregate it accordingly with subdoucment. So that wherever you push a new student into allStudents with the className mentioned it will be pushed to the students subdocument of the respective className.

Answer 2

Querying with dot notation, like

.find({"classes.students":"student8"})

will check the students field of each object in the classes array, returning the documents containing the specific student in any class.

Answer 3

Thank you for the answers given. Just sharing another solution using vanilla Javascript that might be of use to those in similar situations:

  checkStudent = (batch, student) => {
    let result = null
    const inClass = (cls) => {
      if (cls.students.includes(student)) {
        result = cls
        return true
      }
      return false
    }
    batch.classes.some(inClass)
    return result
  }