Something else to consider: Even if you start with a 'to-one' model, if your relationship is mutable, then your relationship is really to-many over time. Serial

Something else to consider: Even if you start with a 'to-one' model, if your relationship is mutable, then your relationship is really to-many over time. Serial 'to-one' relationships end up being to-many anyway. For example, you start off saying 'customers should have an email address'. And you build a lot of stuff on the assumption that customers have a single email address. But then customers insist that they want to be able to change their email address. And you'll find yourself in a better place if handle that by making email addresses into a collection, adding another email address to the customer, and changing which one is marked as 'current', than if you just overwrite the single email address.