如何在Grails / GORM中将Domain Class模型对象从一个派生类更改为另一个派生类 [英] How to change a Domain Class model object from one derived class to another in Grails/GORM

问题描述

给定以下Grails GORM Domain Classes并使用每个层次继承：

  class Book {
 static belongsTo = [parent：Parent] 
字符串标题
} 
 
抽象类父类{
 static hasMany = [books：Book] 
} 
 
 class A extends Parent {
 String asset 
} 
 
 class B extends Parent {
 String asset 
} 
  

假设我从数据库中检索了A类的一个实例。我想将它转换为B类的一个实例。什么是grails惯用的方式来做到这一点？

没有hasMany关系，我只需删除A并创建一个新的B.但我不想要通过大量的书籍和更新他们的parent_id字段指向新的B的开销。

在引擎盖下，我基本上只想执行SQL UPDATE将数据库字段parent.class从A更改为B.那么在GORM / Grails中执行此操作的建议方式是什么？

解决方案

看起来像是设计缺陷的标志。

你可以试着用继承来代替继承 - 优先于类继承的对象组合。 （Gang of Four 1995：20） - 从 Parent 提取a，比如 HasAsset 接口，以及添加对 HasAsset 的引用。

Groovy代表团可以提供帮助。

Given the following Grails GORM Domain Classes and using table-per-hierarchy inheritance:

class Book {
  static belongsTo = [ parent: Parent ]
  String title
}

abstract class Parent {
  static hasMany = [ books: Book ]
}

class A extends Parent {
  String asset
}

class B extends Parent {
  String asset
}

Say I have retrieved an instance of class A from the database. I want to convert it to an instance of class B. What is the grails idiomatic way to do this?

Without the hasMany relation, I would just delete A and create a new B. But I don't want the overhead of going through a large number of Books and updating their parent_id fields to point to the new B.

Under the hood, I essentially just want to perform an SQL UPDATE to change the database field parent.class from A to B. So what's the recommended way to do this in GORM/Grails?

解决方案

Looks like a sign of design flaw.

You could try to replace inheritance with aggregation - "Favor object composition over class inheritance." (Gang of Four 1995:20)" - extract a, say, HasAsset interface from Parent and add a reference to HasAsset.

Groovy delegation could help.

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