如何检查对象是否已保存到数据库

Question

I am using Spring JDBC and I want to create a method which checks if it have been persisted to the database yet or not, meaning that would have gotten an auto incremented key.

I have a field named id of type int. Since primitive ints are auto assigned 0 if no other values is given I could create a method that checks if id == 0 but then I must ensure that my primary key sequences starts on 1.

I could also create a method in the repository to check if the object was saved, however I do not want to do a query for this.

Last option that I could think of was to change the type of the id to Integer and then check if it was null but I don't know if that is a good way to do it.

Are there a way to have a method in the domain layer to do this?

Answer 1

Changing the type of id from int to Integer is considered a good idea: persistence frameworks such as Hibernate and JPA encourage it (see this example from java.net and this one from adam-bien.com ).

Using an object type allows you to quickly tell an unpersisted object from a persisted one just by checking whether id == null . Just for completeness:

• id == null : unsaved object, perform an INSERT
• id != null : object already saved, perform an UPDATE

This way, you don't have to treat 0 (or any other integer value) as a special case meaning uninitialized ; you already have null , which communicates nicely the idea of special case.

Adding a business key

An even better approach, suggested in this article , is to use a business key in additon to the database-managed ID. The business key is an entity identifier which, unlike the auto-generated primary key ( surrogate key ), has some meaning to the user ( natural key ).

You should then override the equals method to compare two objects using the business key, and not the auto-generated primary key.