This is something that many may already use as a best practice, but if not it’s something simple and convenient to add to your repertoire. Sometimes you may have a model that requires additional information if a certain condition is met. For example, I may require a user to add more information about themselves if they wish to be listed publicly, whereas I would not if they do not wish to be listed. By combining ActiveSupport’s Object#with_options and ActiveRecord’s conditional validations, we can implement this behavior in a straightforward and readable manner (assuming here that there is a boolean field called “listed” in the database that is exposed as a checkbox or similar to the user):

class User < ActiveRecord::Base
  # Our standard validations
  validates_presence_of :login
  validates_uniqueness_of :login

  # Validations for listed users
  with_options :if => :listed? do |l|
    l.validates_presence_of :email
    l.validates_length_of :description, :minimum => 100
  end
end

It’s a simple technique that piggybacks off of Rails’s automatic construction of existence query methods (in this case, listed?) for fields in the database combined with the mapping power of with_options and standard conditional validations.