在Grails中保存关联的域类 [英] Saving associated domain classes in Grails

问题描述

我正在努力争取在Grails上获得正确的关联。假设我有两个域类：

  class Engine {
字符串名称
 int numberOfCylinders = 4 
 static constraints = {
 name（blank：false，nullable：false）
 numberOfCylinders（范围：4..8）
} 
} 
 
 class car {
 int year 
字符串品牌
引擎引擎=新引擎（名称：默认引擎）
静态约束= {
引擎（可空：假）
品牌（空白：假，可空：假）
年（可空：假）
} 
} 
  

这个想法是用户可以在不创建引擎的情况下创建汽车，并且这些汽车会获得默认引擎。在CarController中，我有：

  def save = {
 def car = new Car（params）
 if（！car.hasErrors（）&& car.save（））{
 flash.message =Car saved
 redirect（action：index）
} else {
 render（view：'create'，model：[car：car]）
} 
} 
  

当试图保存时，我在Car.engine字段上得到一个空值异常，所以显然不会创建和保存默认引擎。我尝试手动创建引擎：

  def save = {
 def car = new Car（params）
 car.engine =新引擎（名称：默认引擎）
 if（！car.hasErrors（）&& car.save（））{
 flash.message =Car saved 
 redirect（action：index）
} else {
 render（view：'create'，model：[car：car]）
} 
} 
  

也没有工作。 Grails是否无法保存关联的类？我怎么能实现这样的功能？

解决方案

我试图保存汽车时得到的例外是

非空属性引用null或
瞬间值

显然，当试图保存时引擎是空的，但为什么？结果你必须这样做：

  def car = new Car（params）
 car.engine = new Engine（由于引擎不会运行不属于一辆车，你不会得到级联保存/更新/删除，这在我的情况是好的。解决办法是手动保存引擎，然后保存汽车。
 
I'm struggling to get association right on Grails. Let's say I have two domain classes:
class Engine {
    String name
    int numberOfCylinders = 4
    static constraints = {
        name(blank:false, nullable:false)
        numberOfCylinders(range:4..8)
    }
}

class Car {
    int year
    String brand
    Engine engine = new Engine(name:"Default Engine")
    static constraints = {
        engine(nullable:false)
        brand(blank:false, nullable:false)
        year(nullable:false)
    }
}
The idea is that users can create cars without creating an engine first, and those cars get a default engine. In the CarController I have:
def save = {
    def car = new Car(params)
    if(!car.hasErrors() && car.save()){
        flash.message = "Car saved"
        redirect(action:index)
    }else{
        render(view:'create', model:[car:car])
    }
}
When trying to save, I get a null value exception on the Car.engine field, so obviously the default engine is not created and saved. I tried to manually create the engine:
def save = {
    def car = new Car(params)
    car.engine = new Engine(name: "Default Engine")
    if(!car.hasErrors() && car.save()){
        flash.message = "Car saved"
        redirect(action:index)
    }else{
        render(view:'create', model:[car:car])
    }
}
Didn't work either. Is Grails not able to save associated classes? How could I implement such feature?
解决方案
For what is worth, I finally nailed it.

The exception I got when trying to save a car was 

  
not-null property references a null or
  transient value
It was obvious that the engine was null when trying to save, but why? Turns out you have to do:
def car = new Car(params)
car.engine = new Engine(name: "Default Engine")
car.engine.save()
Since engine doesn't belongs to a Car, you don't get cascade save/update/delete which is fine in my case. The solution is to manually save the engine and then save the car.

                        
这篇关于在Grails中保存关联的域类的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！