Sets read only to field non nil in MongoId

test

> user = User.new
> user.username
=> nil
> user.username = 'kira'
=> "kira"
> user.username = 'kira'
=> "kira"
> user.username = 'Light'
Mongoid::Errors::Validations:
message:
  Validation of User failed.
summary:
  The following errors were found: username is readonly
resolution:
  Try persisting the document with valid data or remove the validations.
> user.save
=> true
> user.update!(username: 'another')
Mongoid::Errors::Validations:
message:
  Validation of User failed.
summary:
  The following errors were found: username is readonly
resolution:
  Try persisting the document with valid data or remove the validations.

user.rb

class User
  include Mongoid::Document
  include Mongoid::Timestamps

  field :username, type: String
  field :actived, type: Boolean, default: true

  # set readonly when the field is not more nil
  def username=(new_value)
    msg_error = 'is readonly'
    is_readonly = !self.username.nil? && self.username != new_value
    self.errors.add(:username, msg_error) if is_readonly && !self.errors.added?(:username, msg_error)
    raise Mongoid::Errors::Validations, self if is_readonly

    super(new_value)
  end
  
  # field with default
  def actived=(new_value)
    msg_error = 'is readonly'
    is_readonly = !self.actived.nil? && self.actived != new_value
    is_readonly &&= self.actived != self.fields[:actived.to_s].default_val # check if is different of default
    self.errors.add(:actived, msg_error) if is_readonly && !self.errors.added?(:actived, msg_error)
    raise Mongoid::Errors::Validations, self if is_readonly

    super(new_value)
  end
end