POJO with other POJO references.

Question

I am working on a API to access data stored in a system. The system contains things like people, appointments and procedures associated with those appointments. My application will strictly be read-only.

I am using Spring w/ RowMapper to build objects such a "Person", "Appointment" and "Procedure". I have a DAO for each element. (ie: PersonDAO.getById(), PersonDAO.getByName(), ..).

The issue comes in that Appointment has a reference to a Person object. An it would be nice in the Person object to have a reference to that Person's appointments, but if I begin to load these it becomes a circular reference.

So, I guess my question is the right way to handle this just put the references (Ids) in the POJOs and then have the business layer(?) just make the right calls to get the information? Or is it ok to somehow pass around a reference to the DAO in the actual POJO so that I can lazily load the object objects when referenced? But then how do you handle the circular reference? When I have a Person and I lazily load all their appointments, those appointments will also have a Person associated with them. When I load this Person it could potentially have difference information from the Person I am loading Appointments for.

Person (object x) lazily load -> Appointments could lazily load Person (object x').

Since Person could have changed by the time I went to lazily load their appointments. I really need the Person object in Appointment to refer back to the same Person object.

I'm getting all caught up on this. I know I could just "make it work", but I want to try and find a good solution. I was thinking about using hibernate for this, but thought it was really just overkill. Maybe it isn't.

Answer 1

You're describing a bidirectional association, which Hibernate has specific (and generally very good) support for.

Read up on how to do it in the docs.

Rolling this by hand is going to be quite fiddly and bug-prone. I wouldn't recommend it. Use the the power of ORM tools like Hibernate, that's what they're there for.

Answer 2

Extending the suggestion of using Hibernate, I would recommend checking out the JPA annotation support that Hibernate supports ( I believe it's part of the J2EE spec). You can annotate your classes with a @ManyToMany annotation. Check out these docs:

https://www.hibernate.org/397.html