Entities can be related to each other in different ways. A one-to-one relation is one where each individual entity may be related to another, and this other one, if it exists, is directly related to the first. Examples include the following:
A relation from a person to their address, assuming that no two people share the same address
Additional details for an order
A person and their pet
A country and its head of state
A one-to-one relation is easy to represent in domain model terms: each of the entities has a reference to the other. Only one side can be made required; otherwise, we would have a problem: which one comes first? Let's see how we can map this kind of relation in Entity Framework Core.