与JDO和GAE的多对一关系

Question

I have searched all over the site But there even not one example of how to make with jdo a many to one relation ship. https://developers.google.com/appengine/docs/java/datastore/jdo/relationships

but I couldn't find out how to do it.

lets say I have this DB:

Cars(CarID,OwnerID,...)
Owner(OwnerID,...)

And I want to create new entity for a new car - how can i make it?

I dont want to make another entity for an owner if the owner is already on the db but if he is not I like to make new owner.

Thanks for any example code or blog link on how to deal with it.

EDIT: Its seems like you think its bidir relationship. So i think you didnt understand me well. lets say we are not talking about Car and Owner we are talking now on Genre and Song

Genre-dont need to know about Song! Song need to know his Genre!

my classes are:

public class Genre {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
long id;
@Persistent
String genre;

} AND:

public class Song{
long id;
    @Join(name="GENRE_JOIN")
Genre genre;

}

thanks in advance!

Answer 1

All of the GAE docs say refer to the DataNucleus JDO docs for full details, so I don't see why they should provide examples of everything. Since your relation is seemingly (not that you define it) a 1-N BIDIR relation at Owner side, and N-1 BIDIR at Car side then http://www.datanucleus.org/products/accessplatform_3_1/jdo/orm/many_to_one.html The JDO spec would also have ample examples.

Answer 2

class Owner {
    @Persistent(mappedBy = "owner")
    private List<Car> cars;
}

class Car {
    @Persistent
    private Owner owner;
}

Car car = new Car();
//search for the owner.
car.setOwner(owner);