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我有以下实体

class Thesis extends Article {
   String description
   Topic topic
   User assignee
   User supervisor
   ...
   static constraints = {
       supervisor nullable: true
   ...
      supervisor validator: {supervisor ->
         if (supervisor?.id != null && !User.get(supervisor.id)) {
            'not.found'
         }
      }
   }
}

在没有主管的情况下创建和保存新实例就可以了。

但是，当我尝试从现有论文实例中删除（设置为 null）主管时，我最终会遇到 TransientObjectException。我可能遗漏了一些关于数据绑定的内容，因为如果我不将表单数据绑定到 thesisInstance 并且只是将“supervisor”属性设置为 null，它将毫无问题地保存。

def thesisInstance = Thesis.get(id)
thesisInstance.properties = params.thesis
if (thesisInstance.supervisorId == null){
   thesisInstance.supervisor = null
}
thesisInstance.save()

编辑： params.thesis 地图看起来像这样（免责声明：这些是虚拟数据）

thesis.tags.title: ruby
thesis.version: 7
thesis.topic.id: 13
thesis.supervisor.id: 
thesis.status: FINISHED
thesis.id: 18
thesis.title: Implementation of Ruby Killing Machine
thesis.type: BACHELOR
thesis.assignee.fullName: Somebody
thesis.university.id: 1
thesis.grade: A
thesis.supervisor.fullName: 
thesis.thesisAbstract: Implementation of Ruby Killing Machine that will kill people that are filled in an HTML form.
thesis.description: Implementation of Ruby Killing Machine that will kill people that are filled in an HTML form.
thesis.assignee.id: 4

如您所见，未填充 thesis.supervisor.id，因此将 thesisInstance.supervisor 设置为没有 ID 的新用户实例。因此需要“thesisInstance.supervisor = null”

我在 [1] 上发现了完全相同的问题，但没有得到解答。

[1] http://grails.1312388.n4.nabble.com/Data-Binding-and-Null-ifying-problems-in-G1-2-0-td1339929.html

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