DataMapper - Associations

datamapper ruby orm:

Associations are a way of declaring relationships between models, for example a blog Post "has many" Comments, or a Post belongs to an Author. They add a series of methods to your models which allow you to create relationships and retrieve related models along with a few other useful features. Which records are related to which are determined by their foreign keys.

Full article: http://datamapper.org/docs/associations.html