module SimpleForm::ActionViewExtensions::Builder

def simple_fields_for(*args, &block)

end
end
posts_form.input :title
# Here you have all simple_form methods available
f.simple_fields_for :posts do |posts_form|
form_for @user do |f|

Example:
Wrapper for using SimpleForm inside a default rails form. 
def simple_fields_for(*args, &block)
  options = args.extract_options!
  options[:wrapper] = self.options[:wrapper] if options[:wrapper].nil?
  options[:defaults] ||= self.options[:defaults]
  options[:wrapper_mappings] ||= self.options[:wrapper_mappings]
  if self.class < ActionView::Helpers::FormBuilder
    options[:builder] ||= self.class
  else
    options[:builder] ||= SimpleForm::FormBuilder
  end
  fields_for(*args, options, &block)
end