grails（GORM）/ hibernate中的多对多链接表 [英] Many-to-Many link tables in grails (GORM) / hibernate

问题描述

我正在和Grails玩，我发现ORM的东西很乏味，因为我没有完全理解我在做域类时做了什么。我希望有人能让我回到正轨上

考虑以下内容：

测试工作一：许多硬件用于作业许多：一个物理硬件

...类似于传统Order，OrderLine，在大学数据库示例中看到的产品方案

我创建了以下域类

  class Job 
 {
 String jobName 
 String jobDescription 
} 
 $ b $ class HardwareOnJob 
 {
 static hasMany = [作业：作业，physicalHardware：PhysicalHardware] 
 static belongsTo =作业
 
字符串角色
} 
 
类PhysicalHardware 
 {
字符串assetName 
字符串模型
字符串os 
} 
  

我需要问的是，为什么Grails会在我的数据库中为我创建两个额外的表格，而不是使用我定义的链接实体/域类。例如，Grails会在数据库中创建hardware_on_job_job和hardware_on_job_physical_hardware。

使用脚手架控制器，我可以输入一些硬件，输入工作，然后将两者连接在一起。我的问题是为什么它会创建这两个额外的表，而不是使用我指定的域对象（HardwareOnJob）。

任何帮助/指导都会非常感激，因为会坚持看着这个并尝试新事物。顺便说一下，我在grails版本1.2.1上

解决方案

查看 joinTable 关键字其中：

自定义用于单向一对多，多对多和原始集合类型的连接表

以下是用户指南中的示例：

  class Book {
字符串标题
静态belongsTo =作者
静态hasMany = [作者：作者] 
 
静态映射= {
作者joinTable：[name：mm_author_books， key：'mm_book_id'] 
} 
} 
 class作者{
字符串名称
 static hasMany = [books：Book] 
 
 static如果您的图书馆的图书馆名称为：[
 books joinTable：[name：mm_author_books，key：'mm_author_id'] 
} 
 
} 
  pre> 
I'm playing aroud with Grails and am finding the ORM stuff tedious because I don't fully understand what I'm doing when it comes to domain classes. I'm hoping someone can put me back on track

Consider the following

Test Job  One:Many   Hardware Used on Job    Many:One     Physical Hardware

...this is analogous to the classic Order, OrderLine, Product scenario seen in university DB examples

I've created the following domain classes
class Job
{
  String jobName
  String jobDescription
}

class HardwareOnJob
{
   static hasMany = [  jobs:Job, physicalHardware:PhysicalHardware ]
   static belongsTo = Job

   String role
}

class PhysicalHardware
{
  String assetName
  String model
  String os 
}
The question I need to ask is why does Grails create me two extra tables in my database rather than using the link entity/domain class I've defined. For instance Grails creates hardware_on_job_job and hardware_on_job_physical_hardware in the database. 

Using the scaffolded controllers I can enter some hardware, enter a job and then enter link the two together. The question I have is why does it create these two extra tables rather than use the domain object (HardwareOnJob) I've specified. 

Any help/guidance would be very much appreciated as going nuts looking at this and trying new things. Btw I'm on grails version 1.2.1
解决方案
Have a look at the joinTable keyword which:

  
Customizes the join table used for undirectional one-to-many, many-to-many and primitive collection types
Here is the example from the user guide:
class Book {
    String title
    static belongsTo = Author
    static hasMany = [authors:Author]

    static mapping = {
        authors joinTable:[name:"mm_author_books", key:'mm_book_id' ]
    }
}
class Author {
    String name
    static hasMany = [books:Book]

    static mapping = {
        books joinTable:[name:"mm_author_books", key:'mm_author_id']
    }

}


                        
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