Set a field 'unique' ONLY within a user (mongoose)

Question

I have a standard mongoose User Scheme, and a 'Uniform' scheme that holds dress items in the following way:

mongoose.Schema({
    user: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
    items: { type: [String], unique: true }
})

Thing is, I want the items in the 'items' array to be unique only in the scope of the user. Meaning that I don't want a user to have serval uniforms with the same item. But I don't have any problem with other users having the same items as my user.

The current scheme prevents ANY uniform to have the same values inside 'items', I want it to be just within the scope of the user. Can that even be done?

(In ruby on rails, the line that creates this behavior is: validates_uniqueness_of :items, scope: [:user])

Answer 1

If User and Uniform are different schemas (different collections) then having uniqueness achieved between keys of different collections cannot be done in mongodb.

We can have uniqueness achieved by combining different keys of a same collection using Compound Index