DM example of M:M through join table using two relationships.

1_example.rb

class DocumentSet
    include ::DataMapper::Resource

    property :id, Serial

    # First, define the immediate (intermediate) relationship.
    has n, :document_maps, :child_key => [:ds_id]

    # Then make another relationship that bounces through that relationship
    # (:through), arriving at model Document.
    has n, :documents, :through => :document_maps, :via => :document
end

# This is the intermediate model.
class DocumentMap
    include ::DataMapper::Resource

    property :id, Serial

    property :ds_id, Integer, :required => true, :index => true
    property :document_id, Integer, :required => true, :index => true

    belongs_to :document_set, :child_key => [:ds_id]
    belongs_to :document
end

# Model you want to end up at.
class Document
    include ::DataMapper::Resource

    property :id, Serial

    # Not necessary, but useful for going backwards.
    has n, :document_maps, :child_key => [:document_id]
end


# Then run something like:
DocumentSet.first.documents