Phusion Passenger error 'Unknown key(s): through'

My rails applic开发者_运维技巧ation runs fine on my local testing server, but when I deploy to my production (Passenger) server, I get the following error:

Error message:
  Unknown key(s): through

Exception class:
  ArgumentError

Why might this be happening? The code can't be inherently flawed, since everything works fine on the development server. The offending piece of code is very simple:

class PageEvent < Event
   belongs_to :page, :through => :calendar
end

where

class Event < ActiveRecord::Base
  belongs_to :calendar, :polymorphic => true
end

and

class Calendar < ActiveRecord::Base
  has_many :events, :dependent => :destroy, :inverse_of => :calendar, :as => :calendar, :class_name => "SpecialEvent"
end

I can't say what is happening that it appears to be working in dev, but there is no belongs_to :through.

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to