lib/formula.rb
def formula_fields_for(record_or_name_or_array, *args, &block)
      options = args.extract_options!
      options[:builder] ||= @@builder
      fields_for(record_or_name_or_array, *(args << options), &block)
    end
def formula_fields_for(record_or_name_or_array, *args, &block)
      options = args.extract_options!
      options[:builder] ||= @@builder
      fields_for(record_or_name_or_array, *(args << options), &block)
    end
Generates a wrapper around fields_for with :builder set to FormulaFormBuilder.

 Supports:

 * f.formula_fields_for(@user.company)
 * f.fieldsula_for(@user.company)

 Equivalent:

 * f.fields_for(@user.company, :builder => Formula::FormulaFormBuilder))

 Usage:

   <% f.formula_fields_for(@user.company) do |company_f| %>
     <%= company_f.input :url %>
     <%= company_f.input :phone %>
   <% end %>