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?