How to generate a value for a column in a JPA entity, while querying the database? - spring

Question

I have an entity that looks like this:
#Entity
#Table(uniqueConstraints={#UniqueConstraint(columnNames={"slug"})})
public class BlogPost {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#Column
private String title;
#Column
private String slug;
}
I would like to generate the value of slug before persisting by doing the following:
Transforming the title from e.g. Blog Post Title to blog-post-title
Making sure that blog-post-title is unique in table BlogPost, and if it's not unique, I want to append some suffix to the title so it becomes e.g. blog-post-title-2
Since I need this on a lot of entities, my original idea was to create an EntityListener which would do this at #PrePersist. However, documentation generally states that I should not call EntityMan­ager or Query methods and should not access any other entity objects from lifecycle callbacks. I need to do that in order to make sure that my generated slug is indeed unique.
I tried to be cheeky, but it is indeed very hard to autowire a repository into an EntityListener with Spring anyway.
How should I best tackle this problem?
Thanks!

Answer 1

Both OndrejM and MirMasej are definitely right that generating a slug would not be something to be done in an Entity. I was hoping EntityListeners could be a little "smarter", but that's not an option.
What I ended up doing is using aspects to accomplish what I wanted. Instead of "hooking" into entities, I am rather hooking into save method of CrudRepository.
First, I created an annotation so I can recognize which field needs to be sluggified:
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public #interface Slug {
/**
* The string slug is generated from
*/
String source() default "title";
/**
* Strategy for generating a slug
*/
Class strategy() default DefaultSlugGenerationStrategy.class;
}
Then, I created an aspect which is something like this:
#Aspect
#Component
public class SlugAspect {
... // Removed some code for bravity
#Before("execution(* org.springframework.data.repository.CrudRepository+.save(*))")
public void onRepoSave(JoinPoint joinPoint) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
Object entity = joinPoint.getArgs()[0];
for (Field field: entity.getClass().getDeclaredFields()) {
Slug annotation = field.getAnnotation(Slug.class);
if (annotation != null) {
CrudRepository repository = (CrudRepository) joinPoint.getTarget();
Long count = 0L;
SlugGenerationStrategy generator = (SlugGenerationStrategy)annotation.strategy().newInstance();
String slug = generator.generateSlug(slugOrigin(entity));
if (id(entity) != null) {
Method method = repository.getClass().getMethod("countBySlugAndIdNot", String.class, Long.class);
count = (Long)method.invoke(repository, slug, id(entity));
} else {
Method method = repository.getClass().getMethod("countBySlug", String.class);
count = (Long)method.invoke(repository, slug);
}
// If count is zero, use the generated slug, or generate an incremented slug if count > 0 and then set it like so:
setSlug(entity, slug);
}
}
}
}
I put the code on github (though it's still just a proof of concept) if anyone is interested at: https://github.com/cabrilo/jpa-slug
It relies on having CrudRepository from Spring Data and having these two methods on a repo: countBySlug and countBySlugAndIdNot.
Thanks again for the answers.

Answer 2

The most straightforward solutions seems to make a check before setting the value of the title. It would mean however that the logic of calculating the slug would be outside of the entity and both would come from outside.

Answer 3

You have to think of an entity as a plain object without any connection to the database - this is the idea of ORM. However, you may pass a reference to EntityManager or DAO as an additional argument to a setter method, or somehow inject a reference to it. Then you may call a query directly from the setter method. The drawback of this solution is that you need to always provide EntityManager, either when you set title, or when you create/load the entity.
This is the best object oriented way of solving this problem.