[SOLVED] On which side must I define a @JoinColumn annotation for a @OneToOne relationship like this?

On which side must I define a @JoinColumn annotation for a @OneToOne relationship like this?

Say I have a relationship like figure 1 (@OneToOne between Customer and AtmCard, mandatory on one side but not the other).

Am I correct to say that to enforce the mandatory aspect of the Customer on the AtmCard side, I need to future define a @JoinColumn(nullable = false) together with my @OneToOne annotation like in figure 2?

What happens if I don't define this @JoinColumn annotation with optional set to false? What exactly does @JoinColumn(nullable = false) do, and why should I apply it to all @OneToOne and @ManyToOne relationships only?

For reference, figure 3 shows my annotation mappings on the Customer side. Thanks!

Solution

You will find a pretty good explanation here and in this tutorial

But generally you can achieve bidirectionality in your entites(meaning you can do customer.getAtm() or atm.getCustomer()) without having to create 2 columns with FK in both tables (JPA will use joins to get something). You would usually add nullable false on the child entity (ATM) as an ATM can't exist wihout a customer, but a customer could exist without an ATM (that being the case for real world, unless your business is different then you should force it as parent as well)

We usually add these types in order to help us keep the DB contents valid