While not always the "best" approach, Rails offers what's called a Polymorphic belongs_to association. It prevents you from defining a foreign key in the database because the xxx_id column is referencing an id in one of many possible tables while another column designates the name of that table's model, but it makes the relationship more explicit in Rails. Also, it restricts the model to only belong to one of the other models, as opposed to belonging to one or more, as it would happen using the setup of multiple foreign keys without some additional db magic.
