Objectify 4.0中的实体层次结构 [英] Entity hierarchies in Objectify 4.0

问题描述

有没有办法如何定义一个实体层次结构，使其能够查询特定的子类？考虑下面的情况。让我们有抽象Base类，它定义了公共属性和具体的子类A和B.

  class abstract Base {
 .. 。
} 
 
 class A扩展基础{
 ... 
} 
 
 class B扩展基础{
 .. 。
} 
  

我想按以下方式运行示例查询。

$ b

• 检索类型A和B的所有实体
  
  

    Base base = this.objectify.load（）。type（Base.class）.list（）; 
    




• 检索A类型的所有实体

    Base base = this.objectify.load（）。type（A.class）.list（）; 
    




• 检索B类型的所有实体

    Base base = this.objectify.load（）。type（B.class）.list（）; 
    

此外，我们希望将所有实体作为GAE Datastore中的单一类型（Base实体）。

我们尝试使用此处描述的相关实体类的多态层次结构：

https://code.google.com/p / objectify-appengine / wiki / Entities＃Polymorphism

但似乎无法处理存在多个实体子类的情况我不认为 Base 可以是抽象的，但这应该工作：

  @Entity 
 class Base {...} 
 
 @EntitySubclass（index = true）
 class A extends Base {...} 
 
 @EntitySubclass（index = true）
 class B extends Base {...} 
  

如果您希望能够通过多态类型，你必须索引你想查询的类型。

Is there a way how to define an entity hierarchy that enables to query just particular subclass? Consider situation below. Let's have abstract Base class that defines common properties and concrete subclasses A and B.

class abstract Base {
...
}

class A extends Base {
...
}

class B extends Base {
...
}

I would like to run for example queries as follows.

• To retrieve all entities of type A and B

   Base base = this.objectify.load().type(Base.class).list();
  

• To retrieve all entities of type A

   Base base = this.objectify.load().type(A.class).list();
  

• To retrieve all entities of type B

   Base base = this.objectify.load().type(B.class).list();
  

Furthermore, we would like to store all such entities as a single type (Base entity) in GAE Datastore.

We tried to use polymorphic hierarchy of related entity classes described here:

https://code.google.com/p/objectify-appengine/wiki/Entities#Polymorphism

But it seems that this is not capable of handling a situation where there are multiple entity subclasses with a common parent.

解决方案

I don't think Base can be abstract, but this should work:

@Entity
class Base { ... }

@EntitySubclass(index=true)
class A extends Base { ... }

@EntitySubclass(index=true)
class B extends Base { ... }

If you want to be able to query by polymorphic type, you must index the types that you want to query by.

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