Hi Hari,
I don't follow what you're saying in #1 below.
#2 - This is just the way I have always modeled classes using JPA. In a
relational database, the @ManyToOne would cause the key of the owning
entity to be stored in the row. In this case, the "Rating" table would
have a column to hold the Person_Id. It's unclear to me how this works
with the DataNucleus datastore in GAE. Here's an example of a similar
relationship that is supposed to work in GAE:
http://gae-java-persistence.blogspot.com/2009/10/creating-bidrectional-owned-one-to-many.html
Obviously, the difference is that I have another @ManyToOne that
references another entity type.
I've always felt like I had a firm handle on JPA, but using it with GAE
has been a very discouraging endeavor, so far.
Thanks for the feedback,
Dave
On 10/04/2010 11:56 PM, Hariharan Anantharaman wrote:
Hi,
I have encountered similar error using JDO long time back. I firmly
believe that if you use @ManyToOne or something it means a
relationship amongst them.
Though I don't remember exactly what I did ,Coupleof observations
1. In the rating class. Add annotation of rederenced by attribute in
annotation. Or a similar annotation needs to be done in Book class.
2. Person has a list of rating and rating again has a person. Are you
planning to query by rating entity independently? I think this design
tells JPA that a person indirectly owns a book.
Thanks
Hari
On Oct 5, 2010 2:54 AM, "Dave Hicks" <[email protected]
<mailto:[email protected]>> wrote:
I'm using JPA, and I'm having a really tough time getting around
this problem. It seems that when I try to refer to an object by
way of a @ManyToOne, somehow DataNucleus wants to assume some kind
of ownership and throws an exception.
I have three kinds of objects: Person, Book, Rating. Person and
Book objects are independent of one another and created
independently. When a person rates a Book, a new Rating object is
created and added to a set of Ratings owned by the Person. The
Rating object has a reference to the Book. Here are the classes
(each extends a superclass that contains the @Id element):
class Person extends BasicEntity {
private String email;
@OneToMany(mappedBy="person", cascade=CascadeType.All,
fetch=FetchType.LAZY)
private Set<Rating> ratings;
}
class Book extends BasicEntity {
private String name;
}
class Rating extends BasicEntity {
@ManyToOne(fetch=FetchType.LAZY)
private Person person;
@ManyToOne(fetch=FetchType.LAZY)
private Book book;
private Integer value;
}
I get the following exception when I attempt to add such a Rating
to a Person:
org.springframework.orm.jpa.JpaSystemException: Detected attempt
to establish Person(1)/Rating(3) as the parent of Book(2) but the
entity identified by Book(2) has already been persisted without a
parent. A parent cannot be established or changed once an object
has been persisted.
What gives? Is it not permitted to refer to objects in this way?
I've been using this technique in JPA for years.
Thanks,
Dave
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