Friday, October 29, 2010

Some Hibernate Lessons

I have started using Hibernate in my project and it turns out to be more difficult for a beginner than I thought, especially when the domain model is a bit complicated. But I guess it is worthwhile to do the learning and fix the problems along the way because it will be even more problematic if I choose to brew a layer like that by myself.

So, here are some of the lessons I learned.

1) Inheritance: I chose the 3rd strategy, subclass per table because it is the most beautiful solution to my problem. The lesson I learned is that I don’t need to add an extra ID to the subclass since it will use the parent class’s ID anyways. But for the database table schema, you should have an ID for the subclass table.

2) One-to-many mapping: it is a bit similar to 1) except that now it is a foreign key reference that you don’t need to put into the many-to-one end POJO definition. For example, we have a parent p and a child c, p contains a set of c and c contains a single p, so this is a bi-directional one-to-many mapping. In the tablec, there should be a column like pid that references id column in tablep. But you don’t need to have a property called pid in the c mapping file. Otherwise, you will see duplicated mapping error. You don’t need to have an attribute p_id with getter and setter defined in c POJO either. Otherwise, you will see a null value returned from c.getPId() method.

Here are two links for examples:


subclass per table

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