JDO的Entity關係

來源：https://developers.google.com/appengine/docs/java/datastore/jdo/relationships
物件的關係
owned(可看做是composited的關連，也就是強關聯)
單向一對一關係(unidirectional one-to-one ): 僅只要在Parent中放Child object 即可。
雙向一對一的關系bidirectional one-to-one:在child中加入parent的物件並給annotation( @Persistent(mappedBy = "contactInfo"))其中contactInfo為單向一對一中parent所放置child的field name.

在物件關系中，需注意
. If you do not access the child object on a parent object, the entity for the child object is never loaded. If you want to load the child, you can either "touch" it before closing the PersistenceManager 。

or using

 @Persistent(defaultFetchGroup = "true")
    private ContactInfo contactInfo;

※由於Entity group 的關聯是在create object時建立的，所以在建立物件時，請於關聯中建立，而不要分別建立物件後，再給關係。

一對多的關系Owned One-to-Many Relationships
與一對一類似，僅是替換成Collection of the related class。
而雙向的的方式也是child中加入parent的物件中利用@Persistent(mappedBy ="")

App Engine does not support join queries(can query a property of an embedded class)

※若是collection要排序,則加入annotation,  @Order(extensions = @Extension(vendorName="datanucleus", key="list-ordering", value="state asc, city asc"))

Unowned One-to-One 

Many-to-Many Relationships

transactions