> But in most software systems I've worked with, Many-To-Many relationships are often the most complex, the most difficult to reason about, and the most difficu

> But in most software systems I've worked with, Many-To-Many relationships are often the most complex, the most difficult to reason about, and the most difficult to maintain, because they introduce a third object -- the Relationship itself. From the database perspective, the one-to-many model already requires introducing the third Relationship object. If you have "persons", and you have "pets", you're probably going to end up with "person_pets". Yes, for a one-to-many you can certainly just stick an "owner" column in "pets" instead of having a separate "person_pets" table (ideally called something more sensible like "pet_owners"), but splitting it up doesn't really entail much more effort.