Question

I am still not a stage where I feel completely comfortable with JPA.

Right now I am torn between using relationship annotations or simply retrieving the related objects when I need them with queries.

For instance I have a User who owns Projects. I can use a onetomany relationship and retrieve the projects in an object fashion way OR I can simply query the user’s projects when I need them.

The latter solution involves more code but somehow I have more freedom and control over what I want to do, or at least it is my impression. The former will obviously take care of quite a fair amount of boiler plate code such as create/delete/update of objects, but there are quite a few tricks to learn along the way.

I would greatly appreciate if someone could come up with a simple rule of thumb on when to use relationship annotations in JPA, preferably based on her/his experience.

Thanks,

Thomas

Answer 1

The basic rule is that you should create a relationship when you need it.

For one-to-one and many-to-one relationships it’s quite easy: you need them almost always. For example, when you display Project information you almost always need to display its owner information as well, thus creating a relationship is a good choice here.

One-to-many and many-to-many relationships require more care, since overuse of them can cause performance problems.

My personal rule of thumb is the following: if you don’t need to display all Projects of User at once without pagination or filtering (or at least don’t need to do it often), don’t create a relationship. Otherwise you can create it (for example, you usually need all OrderLines of Order at once, thus you need a one-to-many relationship in this case).