JPA -- Best way to refresh a List association?

Question

Hi,
I need to refresh a OneToMany association.
For example, I have two entities: Header & Detail.
@Entity
@Table(name="HEADERS")
public class Header implements Serializable {
    @OneToMany(mappedBy="header")
    private List<Detail> details;
@Entity
@Table(name="DETAILS")
public class Detail implements Serializable {
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="HDR_ID", referencedColumnName="HDR_ID")
    private Header header;
}So, I fetch the Header along with all its Details.
At a later point of time, I know that some Detail rows in the database have been changed behind my back. I need to re-fetch the list of Details. What should I do?
1. I could add a cascade parameter to the @OneToMany association. I could specify:
@OneToMany(mappedBy="header", cascade={CascadeType.REFRESH})Then I could run:
entityManager.refresh(header);The trouble is that, since all the Details are already in the cache, the cached entities will be returned, not the ones fetched from the database. So, I won't refresh a thing. A query will be sent to the database indeed, but I will get the cached (i.e. stale) entities. I don't know of a way to specify something like
setHint(TopLinkQueryHints.REFRESH, HintValues.TRUE)dynamically for associations, so that the values in the cache would be replaced with the ones fetched from the database.
2. I could try to turn off the caching for the while Entity class. The trouble is that for some reason this doesn't work (see my other question here JPA -- How can I turn off the caching for an entity? Besides, even if it worked, I don't want to turn off the caching in general. I simply want to refresh the list sometimes.
Could anyone tell me what's the best way to refresh the association?
Best regards,
Bisser

Answer 1

Hi Chris,
First, let me thank you that you take the time to answer my questions. I really appreciate that. I wish to apologize for my late reply but I wasn't around the PC for a while.
TopLink doesn't refresh an entity based on a view. I will try to explain in more detail. I hope you'll have patience with me because this might be a bit longer even than my previous post. I will oversimplify my actual business case.
Let's assume we have two tables and a view:
create table MASTERS
  (id number(18) not null primary key,
   master_name varchar2(50));
create table DETAILS
  (id number(18) not null primary key,
   master_id number(18) not null,   -- FK to MASTER.ID
   price number(7,2));
create view DETAILS_VW as
  select id, master_id, price
  from details;Of course, in real life the view is useful and actually peforms complex aggregate calculations on the details. But at the moment I wish to keep things as simple as possible.
So, I create Entities for the tables and the view. Here are the entities for MASTERS and DETAILS_VW, only the essential stuff (w/o getters, setters, sequence info, etc.):
@Entity
@Table(name="MASTERS")
public class Master {
     @Id
     @Column(name="ID", nullable=false)
     private Long id;
     @Column(name="MASTER_NAME")
     private String masterName;
     @OneToMany(mappedBy="master", fetch=FetchType.LAZY, cascade=CascadeType.REFRESH)
     private List<DetailVw> detailsVw;
@Entity
@Table(name="DETAILS_VW")
public class DetailVw {
     @Id
     @Column(name="ID")
     private Long id;
     @ManyToOne(fetch=FetchType.LAZY)
     @JoinColumn(name="MASTER_ID", referencedColumnName="ID")
     private Master master;
     @Column(name="PRICE")
     private Double price;
}So, now we have the tables and the entities. Let's assume one master row and two detail rows exist:
MASTER:  ID=1, MASTER_NAME='Master #1'
DETAIL:  ID=1, MASTER_ID=1, PRICE=3
DETAIL:  ID=2, MASTER_ID=1, PRICE=8And now let's run the following code:
// List the initial state
Master master = em.find(Master.class, 1L);
List<DetailVw> detailsVw = master.getDetailsVw();
for (DetailVw dv : detailsVw) {
     System.out.println(dv);
// Modify a detail
EntityTransaction tx = em.getTransaction();
tx.begin();
Detail d = em.find(Detail.class, 2L);
d.setPrice(1);
tx.commit();
// Refresh
System.out.println("----------------------------------------");
em.refresh(master);
// List the state AFTER the update
detailsVw = master.getDetailsVw();
for (DetailVw dv : detailsVw) {
     System.out.println(dv);
}And here are some excerpts from the console (only the essentials):
DetailVw: id=1, price=3
DetailVw: id=2, price=8
UPDATE DETAILS SET PRICE = ? WHERE (ID = ?)
     bind => [1, 2]
SELECT ID, MASTER_NAME FROM MASTERS WHERE (ID = ?)
     bind => [1]
SELECT ID, PRICE, MASTER_ID FROM DETAILS_VW WHERE (MASTER_ID = ?)
     bind => [1]
DetailVw: id=1, price=3
DetailVw: id=2, price=8You see, the UPDATE statement changes the DETAILS row. The price was 8, but was changed to 1. I checked the database. It was indeed changed to 1.
Furthermore, due to the refresh operation, a query was run on the view. But as you can see from the console output, the results of the query were completely ignored. The price was 8, and continued to be 8 even after the refresh. I assume it was because of the cache. If I run an explicit query on DETAILS_VW with the hint:
q.setHint(TopLinkQueryHints.REFRESH, HintValues.TRUE);then I see the real updated values. But if I only refresh with em.refresh(master), then the DetailVw entities do not get refreshed, even though a query against the database is run. I have tested this both in JavaSE and in OC4J. The results are the same.
An explicit refresh on a particular DetailVw entity works, though:
DetailVw dvw = em.find(DetailVw.class, 2L);
em.refresh(dvw);
System.out.println(dvw);Then the console says:
DetailVw: id=2, price=1So, the price is indeed 1, not 8.
If you can explain that to me, I will be really thankful!
Best regards,
Bisser