Circular reference error on #to_json

I am trying to create a json response that can be used to create a
paging grid. Here is my controller code:

def grid_data
      start = (params[:start] || 1).to_i
      size = (params[:limit] || 20).to_i
      sort_col = (params[:sort] || 'id')
      sort_dir = (params[:dir] || 'ASC')

      page = ((start/size).to_i)+1

      @movie_pages = Paginator.new(self, Movie.count, size, page)

      @movies = Movie.find(:all,
                           :select => "id, title, plot, date, genre,
mpaa, directed_by",
                           :limit=>@movie_pages.items_per_page,
                           :offset=>@movie_pages.current.offset,
                           :order=>sort_col+' '+sort_dir)

      return_data = Hash.new()
      return_data[:Total] = @movie_pages.item_count
      return_data[:Movies] = @movies.collect{ |u| {:id=>u.id,
                                          :title=>u.title,
                                          :plot=>u.plot,
                                          :date=>u.date,
                                          :genre=>u.genre,
                                          :mpaa=>u.mpaa,
                                          :directed_by=>u.directed_by} }
      render :json => return_data.to_json
  end

and I am getting an object references itself error with this stack
trace:

C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:39:in `raise_on_circular_reference'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:26:in `encode'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
core_ext/object/misc.rb:32:in `to_json'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:49
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:49:in `map'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:49
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:27:in `call'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:27:in `encode'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:42:in `raise_on_circular_reference'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:26:in `encode'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
core_ext/object/misc.rb:32:in `to_json'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:57
C:/ruby/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/
base.rb:1940:in `map'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:54:in `each'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:54:in `map'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:54
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
core_ext/object/misc.rb:23:in `returning'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json/encoders/core.rb:53
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:27:in `call'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:27:in `encode'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:42:in `raise_on_circular_reference'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
json.rb:26:in `encode'
C:/ruby/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/
core_ext/object/misc.rb:32:in `to_json'
#{RAILS_ROOT}/app/controllers/movies_controller.rb:41:in `grid_data'

Any ideas? I am stumped.

Figured out that this has something to do with the attribute that is a
date. I I take it out, it works just fine. Hopefully I can figure
out the problem with the date.