has_many through relationship

class Job < ActiveRecord::Base
  has_many :requirements
  has_many :required_jobs, :source => :derived_job, :through => :requirements
  has_many :derived_jobs, :source => :required_job, :through => :requirements
end

class Requirement < ActiveRecord::Base
  belongs_to :derived_job, :class_name => "Job"
  belongs_to :required_job, :class_name => "Job"
end

SQL produced on Job.first.derived_jobs
SELECT `jobs`.* FROM `jobs`  INNER JOIN `requirements` ON `jobs`.id = `requirements`.required_job_id    WHERE ((`requirements`.job_id = 1))
#Why is the association producing job_id versus required_job_id in the where clause, do I need to do a custom finder_sql for the relationship?

#Data Structure
#There is a Job in order to be able to do another Job you need to be at a certain level in your current job(s), so a job has many required jobs where the relationship holds the level where you need to be in order to do the current job