How to apply unique constraints only on specific fields in MongoDB

Question

I have a User schema as follows(JavaScript):-

const userSchema = mongoose.Schema({
    name: {
        type: String,
        required: [true, "User must have a name."]
    },
    password: {
        type: String,
        required: [true, "User must have a password."],
        minLength: 8,
        select: false
    },
    email: {
        type: String,
        required: [true, "User must have an email."],
        unique: true,
        lowercase: true,
        validate: [validator.isEmail, "Email should be in correct format."]
    },
    active: {
        type: Boolean,
        default: true,
        select: false
    }
}

I want to make sure that email is unique but only for the documents where active is set to true. Is this even possible or should I make another collection for inactive user?

Thank you.

Answer 1

This is an anti-pattern, because if a user can suddenly become active after he wasn't active so you will have to enforce a logic to make sure his email is still unique.

I would create a unique index on the email field regarding the active state and only in case you need to keep the non-active users somewhere AND you know a user can't become active again (This is important) so create a new collection for inactive users (keep in mind mongo docs are maximum 16mb so make sure you're not creating an array that can grow without bound)