This point is still quite confusing and unclear on the web, so here is the way I solved my problem. This solution is probably not the most optimized one, but it works when creating and updating a master entity. Theory Use a List instead of a Set for your one-to-many relations which should be dynamically … Read more
The combination of @OneToMany(mappedBy=”…”) and @OrderColumn is not supported by Hibernate. This JIRA issue tracks a request to throw a more obvious error message when this invalid combination is used: http://opensource.atlassian.com/projects/hibernate/browse/HHH-5390 I think that this isn’t supported mainly because it is an odd relational pattern. The annotations above indicate that the “one” side of the … Read more
Looks like everyone is answering One-to-many vs. Many-to-many: The difference between One-to-many, Many-to-one and Many-to-Many is: One-to-many vs Many-to-one is a matter of perspective. Unidirectional vs Bidirectional will not affect the mapping but will make difference on how you can access your data. In Many-to-one the many side will keep reference of the one side. … Read more
I believe that the answer you need is a user-defined aggregate, similar to this one: CREATE FUNCTION gc_init(dummy VARCHAR(255)) RETURNING LVARCHAR; RETURN ”; END FUNCTION; CREATE FUNCTION gc_iter(result LVARCHAR, value VARCHAR(255)) RETURNING LVARCHAR; IF result=”” THEN RETURN TRIM(value); ELSE RETURN result || ‘,’ || TRIM(value); END IF; END FUNCTION; CREATE FUNCTION gc_comb(partial1 LVARCHAR, partial2 LVARCHAR) … Read more
To handle One-To-Many relationships in Django you need to use ForeignKey. The documentation on ForeignKey is very comprehensive and should answer all the questions you have: https://docs.djangoproject.com/en/3.2/ref/models/fields/#foreignkey The current structure in your example allows each Dude to have one number, and each number to belong to multiple Dudes (same with Business). If you want the … Read more
To understand this, you must take a step back. In OO, the customer owns the orders (orders are a list in the customer object). There can’t be an order without a customer. So the customer seems to be the owner of the orders. But in the SQL world, one item will actually contain a pointer … Read more
Yes, this is possible. This is a special case of the standard bidirectional @ManyToOne/@OneToMany relationship. It is special because the entity on each end of the relationship is the same. The general case is detailed in Section 2.10.2 of the JPA 2.0 spec. Here’s a worked example. First, the entity class A: @Entity public class … Read more
The meaning of CascadeType.ALL is that the persistence will propagate (cascade) all EntityManager operations (PERSIST, REMOVE, REFRESH, MERGE, DETACH) to the relating entities. It seems in your case to be a bad idea, as removing an Address would lead to removing the related User. As a user can have multiple addresses, the other addresses would … Read more
I think a newer version of hibernate (supporting JPA 2.0) should handle this. But otherwise you can work it around by annotating the collection fields with: @LazyCollection(LazyCollectionOption.FALSE) Remember to remove the fetchType attribute from the @*ToMany annotation. But note that in most cases a Set<Child> is more appropriate than List<Child>, so unless you really need … Read more