DataMapper连接语句

Question

如何使用ruby DataMapper生成带有连接语句的查询？

即

class Model1
    property id, Serial
    has n :Model2s, :parent_key => [ :id ], :child_key => [ :model1_id ]
end

class Model2
    property :id, Serial
    property :model1_id, Integer
    has n :Model3s, :parent_key => [ :id ], :child_key => [ :model2_id ]
end

class Model3
    property :id, Serial
    property :model2_id, Integer
end

现在我希望DataMapper创建一个类似的查询：

SELECT ... FROM model1 INNER JOIN model2 ON model1.id = model2.model1_id INNER JOIN model3 ON model2.id = model3.model2_id;

目前，在访问时，它会使用单个查询延迟加载所有关系，这非常慢。