module NestedFieldsHelper
  def nested_fields_for(f, association, options={}, &block)
    reflection = f.object.class.reflect_on_association(association)
    is_has_one = reflection.macro == :has_one
    options[:nested_partial] ||= "#{is_has_one ? association.to_s.pluralize : association}/fields"

    add = add_nested_fields_path :association => association,
                                 :builder => f.class,
                                 :nested_partial => options[:nested_partial],
                                 :parent_class => f.object.class,
                                 :parent_form => f.object_name

    partial = "nested_fields/#{'singular_' if is_has_one}nested_fieldset"

    locals = {:add_url => add, :association => association, :f => f}

    locals.merge!(options)

    if is_has_one
      associate = f.object.send(association);

      if associate.nil?
        locals[:associate] = reflection.klass.new
        locals[:has_zero] = true
      else
        locals[:associate] = associate
      end
    end

    if block_given?
      render :layout => partial, :locals => locals, &block
    else
      render partial, locals
    end
  end
end