Mongoose schema: Validating unique field, case insensitive

I have a userSchema like so:

var userSchema = new Schema({
    name: {
      type: String
    , required: true
    , validate: [validators.notEmpty, 'Name is empty']
    }
  , username: {
      type: String
    , required: true
    , unique: true
    , validate: [validators.notEmpty, 'Username is empty']
    }
});

The username field is supposed to be unique. Mongoose will throw an error if this username already exists in the database. However, it is not case insensitive, which I need it to be.

Am I right in thinking that the only way to achieve a case insensitive unique check is to write my own validation rule, which will perform a query on the collection? Is it OK to write validation checks like this, creating more connections to the collection? I will need to do something similar for email, too.