多對多關系

Question

我正在grails / Gorm中嘗試多對多關系示例。 下面是到目前為止我嘗試過的代碼。當我嘗試分析不同的情況時，我想知道Gorm休眠如何處理它。

 class Author {
   static hasMany = [books:Book]
   String name
 }

 class Book {
   static belongsTo = Author
   static hasMany = [authors:Author]
   String title
 }

 And in my controller i defined this way in order to add Authors and books.

 def a=new A(name:"ABC")
 def b=new B(title:"Code1")
 a.addToB(b)  
 def a=new A(name:"ABC")
 def b=new B(title:"Code2")
 a.addToB(b)         //It works.

 In the databaselevel it creates

 Table Author        Table Author-Book        Table Book
 id name               id   id                 id    Book
 1  ABC                1     1                 1     Code1
 2  ABC                2     2                 2     Code2

But what i want is the below format:

Table Author        Table Author-Book        Table Book
 id name               id   id                 id    Book
 1  ABC                1     1                 1     Code1
                       1     2                 2     Code2

將作者姓名設置為唯一時，如何實現？

Answer 1

您的域設置很好。 您將顯式創建兩個作者（具有相同的姓名）。 我將您的代碼更改為

def a=new A(name:"ABC")
 def b=new B(title:"Code1")
 a.addToB(b)  
 def a= A.findOrSaveByName("ABC")       // this will attempt to query the db first and only create new record it already doesn't exist
 def b=new B(title:"Code2")
 a.addToB(b)         //It works.