baffled on forms and controllers!!

Hi Chaps,

Having a little issue with my date filter business here, ive got a
date_filter (something i added to my form builder - which works by the
way) in my page, and I want to use it to filter my @articles.

in my page.....

<% form_for @date_filter do |f| %>
    <%= f.date_field :before %>
    <%= f.date_field :after %>
<% end %>

in my ruby class...

class DateFilter
attr_accessor :before, :after

def initialize(attributes)
    attributes.each { |k,v| self.send("#{k}=", v) }
end

end

and in my controller...

  @date_filter = DateFilter.new(params[:date_filter])
    @articles = @results.find(:all,
                                :conditions => [ "created_at >= ? and
created_at <=?", @filter.before, @filter.after],
                                :order => 'created_at desc', :page =>
{:size => 5, :current => params[:page]})

but I am receiving the following error...

ActionView::TemplateError (Called id for nil, which would mistakenly be
4 -- if you really wanted the id of nil, use object_id) on line #24 of
admin/articles/index.html.erb:
21: <h3>Filter by: Date</h3>
22: Between <br/>
23:
24: <% form_for @date_filter do |f| %>
25: <%= f.date_field :before %>
26: <%= f.date_field :after %>
27: <% end %>

Perhaps someone could help me to see whats going wrong...

Many thanks in advance