Bobzone/mapping-kata

Self-learning workshop with JPA and other stuff

Mapping-kata

Things to know

This section contains things you should know while building a similar app or using JPA in general. Obviously this section is nowhere near being complete, treat it more like a scratchpad that illustrates the mind of developer.

1. Don't use @Inheritance ad with PersonalizedEntity here. The superclass is here only to implement JPA-related fields (id), not to model any sort of OOP design. @MappedSuperclass is preferred.

// Don't use @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) here.
// @Inheritance is to be used for modelling the OOP aspects of domain
// @MappedSupperclass is for adding base class persistent properties as if they were declared in them directly
public abstract class PersonalizedEntity {

2. If you will implements CommandLineRunner SpringBoot allows you to implement the run method. You can put code that you want to execute without corrupting the main method.

public class SimpleMappingKataApplication implements CommandLineRunner {

    @Override
    public void run(final String... strings) throws Exception {
        // put code here...
    }

3. Ran into some dependency issues during development:

Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: 
Error creating bean with name 'simpleMappingKataApplication': Unsatisfied dependency expressed through constructor parameter 0; 
nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'contactService' 
defined in file [C:\Users\epiobob\Documents\Dev\simple-mapping-kata\target\classes\com\bobzone\training\service\ContactService.class]:
Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: 
No qualifying bean of type 'com.bobzone.training.repo.ContactRepository' available: expected at least 1 bean which qualifies as autowire candidate. 
Dependency annotations: {}

3a. Resolved with adding @Repository to ContactRepositoryImpl (could have used @Component as well):

@Repository
public class ContactRepositoryImpl extends AbstractRepositoryImpl<Contact> implements ContactRepository {

4. Another exception that happened during development:

Not allowed to create transaction on shared EntityManager - use Spring transactions or EJB CMT instead; 

4a. Given the code in AbstractRepositoryImpl:

public T merge(final T obj) {
        try {
            em.getTransaction().begin();
            T merged = em.merge(obj);
            em.getTransaction().commit();
            return merged;
        } finally {
            em.close();
        }
    }

SpringBoot complains that you want to manually manage the TransactionManager without having additional Config classes. Dead simple fix is to use the @Transactional adnotation instead:

    @Transactional
    public T merge(final T obj) {
        return em.merge(obj);
    }

5. Im looking in my h2 db and it seems there is no relation between entities! WTF IS GOING ON?

        // Those will be persisted, but the foreign key will appear as null
        Contact contact1 = new Contact("Obi-Wan", "Kenobi", "O'Connell Street Lower, Dublin");
        contact1.getPhoneList().add(new Phone("555-055-055"));

        Contact contact2 = new Contact("Qui-gon", "Jin", "Pembroke Cottages, London");
        contact2.getPhoneList().add(new Phone("333-333-333"));

        Contact contact3 = new Contact("Darth", "Maul", "56 Goldbrook sq, Wellington");
        contact3.getPhoneList().add(new Phone("666-666-666"));

        // This solves the problem of null in database
        final Phone phone1 = new Phone("066-66-66");
        contact3.getPhoneList().add(phone1);
        phone1.setContact(contact3);

You have to manually take care of the mapping if you have a bi-directional relation.

Things to look into in future

1. Usage of @Repository adnotation in classes in the project. It is possible that there are no redundant adnotations right now, but it is a detail to look into.