I am writing an ER diagram for a pet spa, with pet being an entity as a superclass with the disjoint connection to the subclasses of either cat or dog. A service they can partake in is a massage, but it is only offered to a pet if it is a senior. Would I represent senior as an attribute of pet? Or is there a better way to represent that only a senior pet can participate in the relationship which connects pet as an entity to the entity of massage?
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