using a Model in a rake task?

I''m trying to use a model in a Rake task like this:

require 'time'

namespace :halo do desc "post to the usertable via "   task :post_user_with_activerecord do     end_time = Time.new     start_time = end_time - 60     until start_time > end_time       start_time = start_time + 15       heartrate = User.new(:user_id => 817, :timestamp => start_time, :value=> rand(5)+70)       heartrate.save     end   end end

But I get the following error rake aborted! uninitialized constant User

When I add require 'User', I get an error saying uninitialized constant ActiveRecord. When I add require 'active_record', I get another error.

Is there something obvious that I'm missing?