Single Repository, one Aggregate

A Repository as defined in Domain Driven Design manages a single Aggregate. An aggregate may contain many entities, and value objects, but will have a single object as its root. Many of the Dao and even now some of the Repository implementations I see do not follow this, they are more likely to have a Repository per entity, than a Repository per aggregate, and of course in some cases this is required for various reasons.

Ok, to start out we need our POM (or you can use Gradle), which configures our dependencies and plugins. We use a starter for Spring Data JPA, which pulls in Spring Data JPA and all of it's suggested dependencies such as Hibernate. We also need a database and a database driver so we configure H2. Since we are inheriting from the Spring Platform BOM we don't need to specify versions as it can configure them for us. We of course want to use Java 8 and specify our Application class so we will be able to run mvn spring-boot:run at the end.


Next Let's configure our application to show the SQL that it is running, this isn't required. You need to put application.properties in src/main/resources.

Now we need to create our Entities, let's start at the entity Bar that is the deepest part of the Aggregate root. It extends AbstractPersistable so that we get an Auto Incrementing or Sequenced id. We also use AbstractPeristable because for our task we require that our entities implement Persistable, as it changes the behavior of save on the repository if your objects are new.

Next let's create Foo, it is much the same, but you'll notice the @OneToOne that specifies CascadeType.ALL. This is important as without it persist and merge won't work.
Alright, let's put together our repository, we could just make a CrudRepository, but let's show off some paging too. You'll notice that you have to pass the Entity and it's Primary Key identifier type to the PagingAndSorting interface, the single method that we specify will find all the Foos by the nested baz property, using a LIKE '%mystring%' query. Spring data will parse this interface and make an implementation for you automatically.

You can create other lastly we do our Application, which is not designed to be a web server (it will exit immediately). The @SpringBootApplication makes Spring Boot able to start the app and scan for components appropriately. We also Enable JPA repositories using the @EnableJpaRepositories. It's main method (not recommended to prepopulate data this way), creates and save several Foos with nested bars, then I demonstrate a way that you can page the saved objects 2 at a time whilst filtering by that like statement, only 3 of the 4 entities saved will return.

The full source is available here.