immutable.rb

class ImmutableValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, _value)
    return unless record.persisted?

    association_key = if record.attribute_names.include?(attribute.to_s)
                        attribute
                      else
                        record.reflections[attribute].foreign_key
                      end

    record.errors[attribute] << 'can\'t be changed' if record.send("#{association_key}_changed?")
  end
end