JpaRepository: using repositories in #PostPersist - spring

Question

I have two entities
class A {
...
Integer totalSum;
Set<B> b;
}
class B {
...
Integer value;
A container;
}
I want a.totalSum to be sum of each a.b.value;
Maybe best solution is view in db, but I want listen changes in BRepository and update A-records.
I do:
#PostPersist
#PostUpdate
#PostRemove
private void recalculateSums(B b) {
AutowireHelper.autowire(this, this.aRepository);
AutowireHelper.autowire(this, this.bRepository);
A a = b.getContainer();
a.setTotalSum(bRepository.sumByA(s));
aRepository.save(s);
}
And in BRepository:
#Query("select sum(b.value) from B b where b.container = :a")
Long sumByA(#Param("a") A a);
And I have error: org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint ["PRIMARY KEY ON PUBLIC.B(ID)"; SQL statement:
insert into b (VALUE, CONTAINER_ID, ID) values (?, ?, ?)
What I'm doing wrong?
If I do
a.setTotalSum(a.getTotalSum()+1);
aRepository.save(s);
All works, but if I do
a.setTotalSum(a.getTotalSum()+1);
aRepository.saveAndFlush(s);
I have the same error.

Answer 1

I can see why you might want to do this but I think it creates a whole load of potential issues around data integrity.
If you want to have the sum of B available for an A without having to load and iterate all B there are three other options which you could implement all of which would be more robust than your proposal.
As you noted, create a view. You can then Map an Entity say, SummaryData, to this view and map a one-to-one from A to SummaryData so that you can do a.getSummaryData().getTotalSum();
Alternatively, you could use the Hibernate specific #Formula annotation which will issue an inline select when the entity is loaded.
#Formula("(select sum(value) from B b inner join A a where b.a_id= a.id and a.id =id
private int totalSum;
Finally, and depending on the capabilities of your database, you could create a Virtual Column and Map a property as you would for any other field.
1 and 3 obviously require schema changes but your app would remain JPA compliant. 2 does not require any schema change but breaks strict JPA compliance if that was important.