A prototype of my next gem.

example.rb

#app/filters/user.rb
FilterIt.create('user') do
  requires :name, :class => String # stop the filter if no name is found on 'data'
  if current_user.admin? # search in each context for one that respond to the method 'current_user' without an error
    # optional will not raise an error if isn't found, if any validation fail it will be simply ignored.
    optional :authorizantion_level, :class => Integer,:in => 0..10
  end
  if current_user.manager?
    optional :org_ids, :class => Array do
      unless (is_creating_user rescue false)
        data.each do |id|
          # RefuseEntry will make :org_ids invalid, because it is optional the 'user' filter will not fail
          # but the output will not have the :org_ids key
          raise FilterIt::RefusedEntry unless current_user.orgs.include?(id)
        end
      end
    end
  end
end

#app/filters/org.rb
FilterIt.create('org') do
  requires :name, :class => String
  optional :users, :class => Hash do
    # if final data is set the filter will use it, so :users will turn into the output of a map of 'user' filter
    final_data = data.map do |user|
      inherit :user, :data => user, :contexts => all_contexts + [{ :is_creating_user => true }] 
    end
  end
end

#app/controllers/orgs_controller.rb
def create
  # :contexts expect an array of objects, if the object is a hash, each key will
  # be transformed in a method, otherwise it will be used by method_missing
  # and if a context respond to the method without error it will be turned into a
  # method for performance
  data = FilterIt.filter(:org, :data => params[:org], :contexts => [self])
  if data
    Org.create!(data)
  end
end