Rails 3 association with order by field with belongs to relation

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.