Prevent change of one field in Rails model

Question

I have two related models - let\'s say Activity and Step. Activity has_many :steps and Step belongs_to :activity which me

Answer 1

You can declare an attribute as read_only with attr_readonly :your_field_name. But this won't create an error if you try to write this attribute, it will fail silently. (This attribute will be ignored for all SQL-Updates)

Another option might be, to write a validation for this case, might look like this:

class Step < ActiveRecord::Base
  validate :activity_id_not_changed

  private

  def activity_id_not_changed
    if activity_id_changed? && self.persisted?
      errors.add(:activity_id, "Change of activity_id not allowed!")
    end
  end
end

persisted? returns true, if this is not a new record and it is not destroyed.

Links:

http://api.rubyonrails.org/classes/ActiveRecord/ReadonlyAttributes/ClassMethods.html#method-i-readonly_attributes

http://api.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-persisted-3F