As of now I would confirm that, and of course I didn't mean "new
school" in any sense dogmatic, it's just different ways of doing
things on different platforms. I'm even going more and more towards
NOT having relationship fields on GAE at all, even unowned in the JDO
sense. Means not even storing keys in a field/array of the parent.
We'd rather have a field like this on the child:
@Persistent
@Extension(vendorName="datanucleus", key="gae.parent-pk",
value="true")
private String parentKey;
Set its parentKey explicitly and look up all children with a key query
as described above. This gets rid of referential integrity hassle
because you don't have to make sure the key is ALSO stored in the
parents list of child keys... you just always get the actual list of
keys and can be sure you've got all of them.
There is more issues with JDO coming ing when removing children from
lists of child objects in detached state you might want to be aware of
(e.g. check out the test project here:
http://code.google.com/p/datanucleus-appengine/issues/detail?id=218 ).
Best,
Fred
On 4 Aug., 06:42, Vikas Hazrati <[email protected]> wrote:
> I guess that makes sense and your example is dot on. I was wondering
> are there real life applications on GAE where people are using JDO/JPA
> relationships. The more the problems that I face with relationships
> and the more I read about it, it seems that unowned is the way to go.
>
> Thanks,
> Vikas
>
> On Aug 4, 12:53 am, Frederik Pfisterer <[email protected]> wrote:
>
> > There are many Use Cases where it makes sense to keep relationships or
> > you at least want to leverage the power of entity groups even with
> > JDO. The school-of-thought is a bit different than in other
> > environments though. While JDO parent to child access is comfortable
> > it makes not the best use of GAE's massive scalability and performance
> > advantages. One example:
>
> > Say you have a user and all his comments are children in his entity
> > group. Now in an old school way you would check how many there are is
> > user.getComments().size() ... well that works but would have to load
> > the entire array from the store. The new school way of doing this
> > would be running a query filtered on keys that is an order of
> > magnitude
> > faster:http://code.google.com/intl/de-DE/appengine/docs/java/datastore/queri...
> > You can always retrieve the parent object (user) because its key is
> > encoded in any of the child keys.
>
> > There are many more examples where you have to think different, but in
> > general: It makes sense to have entity groups where they persist in
> > reality: users - comments, users - addresses, bank accounts -
> > transactions, and so forth...
>
> > Hope this helps,
> > Fred
>
> > On 3 Aug., 20:28, Vikas Hazrati <[email protected]> wrote:
>
> > > Hi Fred,
>
> > > Thanks for your response. This is what I suspected too that the entity
> > > groups might not be able to change. This actually brings me to the
> > > conclusion that we might not want to keep any owned entities at all.
> > > We are in the process of converting a legacy application to GAE and in
> > > the process we have been breaking out relationships due to multiple
> > > problems. I would write about all the problems that we faced and the
> > > workarounds and link it here.
> > > This relationship was probably one of the last bastions that we had
> > > remaining. With what you mentioned this might have to be broken too
> > > since a user can move around departments as a valid business case.
>
> > > So let me ask the community this question, does anyone see the benefit
> > > of maintaining relations at all? using JPA/JDO. From the complex
> > > queries that a normal web app would have you would fall into
> > > situations where you would need to use more and more of unowned
> > > relationships. Is that true? or am I missing something.
>
> > > Regards | Vikaswww.inphina.comwww.thoughts.inphina.com
>
> > > On Aug 3, 10:00 pm, Frederik Pfisterer <[email protected]> wrote:
>
> > > > You have to use unowned relationships and not place the department and
> > > > the user in the same entity group.
> > > > See:http://code.google.com/intl/de-DE/appengine/docs/java/datastore/relat...
> > > > use for UserDepartment:
> > > > private Set<Key> users = new HashSet<Key>();
> > > > and for User:
> > > > private Key userDepartmentKey;
>
> > > > Entity groups cannot be changed once being established.
> > > > If you want the relationships like you have them, you would have to
> > > > change the Key of the User object (it's identity) to place it in the
> > > > entity group of another department, since the department's (parent)
> > > > key is part of the user key like in a path.
>
> > > > Cheers,
> > > > Fred
>
> > > > On 3 Aug., 15:09, Vikas Hazrati <[email protected]> wrote:
>
> > > > > Oops I somehow managed to hit the submit without the full question...
> > > > > Apologies...Let me start again
>
> > > > > I have 2 entities UserDepartment and User, which are defined as
> > > > > follows
> > > > > @Entity
> > > > > public class UserDepartment extends DomainObject<Long,
> > > > > UserDepartment>
> > > > > {
> > > > > @Id
> > > > > @GeneratedValue(strategy = GenerationType.IDENTITY)
> > > > > private Long departmentId;
> > > > > @OneToMany(mappedBy = "userDepartment", cascade =
> > > > > CascadeType.ALL, fetch=FetchType.EAGER)
> > > > > private Set<User> users = new HashSet<User>();
>
> > > > > and
>
> > > > > @Entity
> > > > > public class User extends DomainObject<Long, User> {
> > > > > @Id
> > > > > @GeneratedValue(strategy = GenerationType.IDENTITY)
> > > > > @Extension(vendorName = "datanucleus", key = "gae.encoded-pk",
> > > > > value = "true")
> > > > > private String encodedKey;
> > > > > @Extension(vendorName = "datanucleus", key = "gae.pk-id",
> > > > > value = "true")
> > > > > private Long userId;
> > > > > private String username;
> > > > > @ManyToOne(fetch = FetchType.EAGER)
> > > > > private UserDepartment userDepartment;
>
> > > > > As you would notice that UserDepartment is the parent of the User. Now
> > > > > once I persist the entity group, i want to modify the user so that it
> > > > > belongs to a different department now.
>
> > > > > What is the best way to do that?
>
> > > > > If i remove the UserDepartment from the User and then assign a new
> > > > > UserDepartment to it then I get into the issue of trying to access
> > > > > multiple entity groups in the same transaction.
>
> > > > > Regards | Vikaswww.inphina.com
>
> > > > > On Aug 3, 6:05 pm, Vikas Hazrati <[email protected]> wrote:
>
> > > > > > I have 2 entities UserDepartment and User, which are defined as
> > > > > > follows
>
> > > > > > @Entity
> > > > > > public class UserDepartment extends DomainObject<Long,
> > > > > > UserDepartment>
> > > > > > {
>
> > > > > > @Id
> > > > > > @GeneratedValue(strategy = GenerationType.IDENTITY)
> > > > > > private Long departmentId;
>
> > > > > > and
>
> > > > > > @Entity
> > > > > > public class User extends DomainObject<Long, User> {
>
> > > > > > @Id
> > > > > > @GeneratedValue(strategy = GenerationType.IDENTITY)
> > > > > > @Extension(vendorName = "datanucleus", key =
> > > > > > "gae.encoded-pk", value
> > > > > > = "true")
> > > > > > private String encodedKey;
>
> > > > > > @Extension(vendorName = "datanucleus", key = "gae.pk-id",
> > > > > > value =
> > > > > > "true")
> > > > > > private Long userId;
>
> > > > > > private String username;
> > > > > > @ManyToOne(fetch = FetchType.EAGER)
> > > > > > private UserDepartment userDepartment;
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