Rails 3 association with order by field with belongs to relation

Rails
1

Hi,
I have three model subject, document_types and document.

class Subject < AR
     has_many :documents
end

class DocumenType < AR
    has_many :documents
end

class Document < AR
   belongs_to :subject
   belongs_to :document_types
end

I need to list all the documents under a subject order by

document_type’s start_date.
What is exact syntax I will use in this association to solve this
problem.

Thanks in advance.