Grails：域类中的派生字段
假设我有一个域类branch，它有多个成员：
…
static hasMany = [ members:Member ];
…
现在假设我希望该分支的成员数量随时可用，以便将其显示在list中，并且view 操作，因此将该信息存储到域类本身的变量中也许是个好主意？
…
Integer memberCount = members.size();
static constraints = {
memberCount(editable:false);
}
…
（这是正确的语法吗？） 编辑：这不是正确的语法。我无法评估成员列表的大小，因为它还不存在，并且 grails 会抱怨 size() 不适用于空对象。我还能尝试什么？
然而，由于 memberCount 现在是域类中的一个变量，因此可以在创建 Branch 时为其分配一个值（这是违反直觉的），并且它添加新成员后不会自动更新。
当然也可以通过不同的方式达到预期的结果。我可以操作 /Branch 目录中的 view.gsp 和 list.gsp，添加一些额外的
基本上，我认为我正在寻找的是某种方式来告诉 grails 某个变量是派生的，不应该由用户设置，而是在必要时进行更新。有这样的办法吗？
Suppose I have a domain class branch which has several members:
…
static hasMany = [ members:Member ];
…
Now suppose I want to have the number of members of that branch readily available, to have it displayed in the list and view actions, so perhaps storing that information into a variable in the domain class itself would be a good idea?
…
Integer memberCount = members.size();
static constraints = {
memberCount(editable:false);
}
…
(Is this the correct syntax?) Edit: This is not the correct syntax. I cannot assess the size of the members list, as it doesn’t exist yet and grails will complain about size() not being applicable to null objects. What else could I try?
However, as memberCount is now a variable in the domain class, it is possible to assign a value to it upon creation of the Branch (which is contra-intuitive) and it will not be updated automatically once a new Member is added.
It is of course possible to reach the desired result in a different way. I could manipulate the view.gsp and list.gsp in the /Branch directory, add some additional <td>s there etc. But that does not seem very elegant to me.
Basically, I think what I am looking for is some way to tell grails that a certain variable is derived, should not be setable by the user, but update whenever necessary. Is there such way?
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您可以将任何您不想保留的属性添加到
transients静态列表中：请参阅 手册中的此页
另外，这个 StackOverflow 问题回答了同样的问题
另外，还有一种更好的方法来处理派生属性可能是使用 派生GORM的属性特点
You can add any property you don't want persisted to the
transientsstatic list:See this page in the manual
Also, this StackOverflow question answers the same question
Also, a better way to do derived properties may be to use the Derived Properties feature of GORM