how to write this in rails ORM method ?

gistfile1.txt

SELECT et.id
FROM examtypes et
LEFT JOIN categoryexams ce ON et.id = ce.examtype_id
LEFT JOIN categoryexamusers ceu ON ce.id = ceu.categoryexam_id
LEFT JOIN categoryusers cu ON cu.id = ceu.categoryuser_id
LEFT JOIN users u ON u.id = cu.user_id
WHERE u.id =71

examtype
has_many :categoryexams

exam
has_many :categoryexam, :dependent => :destroy

category_exam_user
belongs_to :categoryexam
belongs_to :categoryuser

category_exam
belongs_to :exam
belongs_to :examtype
has_many :categoryexamuser
has_many :categoryusers, :through => :categoryexamuser