How to avoid duplicated values?

[Kushagra_Kesav]

Hey @Arie_Levental,

Welcome to the MongoDB Community!

@Arie_Levental

I have this mongoose scheme and for some reason I still manage to have duplicated items in the students array

Based on the schema, it seems there is a “User collection” that contains “User” documents (students).

The “Institution collection” contains “Institution” documents, with each document having an array of student ObjectIds in the student’s field as a reference.

So, to better understand the issue you are facing - Is the same student ObjectId being added multiple times to the students array in an Institution document?

For example:

// Institution document
{
  _id: 1,
  name: "University of MongoDB",
  students: [
    ObjectId("601af221b06858b7b8e35672"), // John's student ID 
    ObjectId("601af221b06858b7b8e35672") // John's student ID added again
  ]
}

To prevent this, you can check if it already exists before adding or using $addToSet operator to only add unique values.

@Arie_Levental

students: [
      {
        type: mongoose.Schema.Types.ObjectId,
        ref: "User",
        unique: true,

The unique:true on the students field does not help here, since that only ensures the field value itself is unique per document, not the array elements.

I hope it clarifies your doubts. In case of any further questions feel free to reach out!

Regards,
Kushagra