GAE one to many Relationship JDO Example

Question

I read GAE doc about JDO but i didn't find such example that fit to my scenario. Please help me.

I have two JDO classes like A and B. "A" class have one to many relationship with "B". This relation is unowed also not dependent. class "A" and "B" has following structure (to make it simple i am not using annotations)

class A {

private String name;
private String id; // primary Key
private List<B> b;

..... getter setter
}

class B {

private Key key; // primary Key
private String id; // this is the attribute of the Class A
private String someData;

}

Now my scenario is that First I created class "A" objects one by one and save it to the data store using Persistent manager. For this storing "List<B> b " will be null bcz i don't have data for it.

After that lets suppose I provide a form that have two fields one for the "id" attribute of the Class "A" and other for "someData" attibute. In this user provide the "id" of one of the stored objects of class "A". When User submit the result i have to create a class "B" object and save it to data store and also have to add it in list of B of its corresponding Class "A" object. I hope you understand what i want to do.

Now my question is that what approach i have to use for maintaining this one to many relationship of JDO classes. Either i have to first retrieve the class "A" object based on the "id" provided in the form and after getting the object i get "list of B" class objects and add a "B" class object in the list and then save the class "A" object( As object is attached to the PM its automatically save) or other approach is that I just create the class "B" object and save it to the data store using PM.

If just creating B class object and saving it to the data store is persistent to the relationship. I simple word when i get the class A objects does "list of B" class object have the all objects linked to class "A" object ( because B class has attribute which is the primary key of class A )

Thanks

Answer 1

It seems to me that the GAE/J documentation at Java | Storing Data | Datastore | JDO | Entity Relationships in JDO | Unowned Relationships should help you here.

Because the relationship between A and B is unowned, all A and B instances are in separate entity groups. Therefore, you will have to perform two separate persistence operations to create/update/delete an instance of B and update the relevant instance of A accordingly.

Does this help?