Using custom form builders with nested models in Ruby on Rails

I'm using nested model form technique from Railscast 197 (ASCIIcast here). But I'm running into a problem with my setup, I need to use a custom form builder for one of my nested models' partials. I've got it working, sort of, by modifiying the link_to_add_fields_helper, like so...

def link_to_add_fields(name, f, association)  
  new_object = f.object.class.reflect_on_association(association).klass.new  
  fields = f.fields_for(association, new_object, :child_index => "new_#{association}", :builder => AnswerFormBuilder) do |builder|
  #was...
  #fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
    render(association.to_s.singularize + "_fields", :f => builder)  
  end  
  link_to_function(name, h("add_fields(this, \"#{association}\", \"#{escape_javascript(fields)}\")"))  
end

So you see by setting both nested models the AnswerFormBuilder I kinda solved my problem, but it's not very elegant since both Questions and Answers don't need them. Additionally I've gotten to a point where I'd like to do an Application wide form builder. Which brings me to my second question, how can I "nest" custom form builders? I.e. have AnswerFormBuilder implement all the methods that the ApplicationFormBuilder would have, plus some other special ones for Answers. Thanks so much, I've looked everywhere and asked on other blogs but no answer yet.

Solution

So I figured it out months later...

def link_to_add_fields(name, f, association, options = {})  
  new_object = f.object.class.reflect_on_association(association).klass.new  
  fields = f.fields_for(association, new_object, :child_index => "new_#{association}", :builder => options[:builder]) do |builder|
    render(association.to_s.singularize + "_fields", :f => builder)  
  end  
  link_to_function(name, h("add_fields(this, \"#{association}\", \"#{escape_javascript(fields)}\")"))  
end

Then when I when I need a custom form builder I just call pass link_to_add_fields a hash with the key :builder and value of the specific builder.

link_to_add_fields "Add Question", f, :questions, :builder => QuestionFormBuilder

If no hash (or no :builder key value pair) is passed to link_add_fields the fields_for method defaults to the regular form builder.