Here's a way to make a relation immutable that doesn't break.

gistfile1.rb

  # Prevent self.user from being changed
  alias_method :old_user_assign, :user=

  def user=(new_user)
    if self.new_record?
      old_user_assign new_user
    else 
      logger.warn "Attempt to modify user of record, id=#{self.id} with user id=#{new_user.id}"
      return false
    end
  end