如何获得最大ID的项目？(How to get item with max id?)

如何获得最大ID的项目？(How to get item with max id?)

我试图用这个查询来获得带有最大id的系统

realm.objects(Entity).filter("@max.id").first

它抛出一个错误，说无法解析查询，所以看起来这不是正确的方法。

如何在Realm for Swift上编写此查询？

I'm tryig to get the ítem with max id using this query

realm.objects(Entity).filter("@max.id").first

It's throwing an error saying that can't parse query so it seems this is not the correct way to do it.

How can I write this query on Realm for Swift?

最满意答案

单独使用过滤器无法实现您所追求的目标，因为它们一次只考虑一个顶级对象。

除了这个概念问题之外，您发布的代码还存在一些问题：

@"@max.id"不是有效的NSPredicate格式字符串 。 NSPredicate格式字符串必须由表达式之间的比较组成，而不是由表达式组成。

必须将@max等集合运算符应用于集合。 在你的例子中，它被应用于一个Entity 。 由于一个Entity不是一个集合，所以谓词是无效的。 不过，将集合运算符应用于Entity上的List属性将是有效的。

像下面这样的东西应该做你以后的事情：

let entities = realm.objects(Entity) let id = entities.max("id") as Int? let entity = id != nil ? entities.filter("id == %@", id!).first : nil

Filters alone cannot do what you're after as they only consider a single top-level object at a time.

Beyond that conceptual issue, there are a few issues with the code you posted:

@"@max.id" is not a valid NSPredicate format string. NSPredicate format strings must be composed of comparisons between expressions, not expressions on their own.

Collection operators such as @max must be applied to a collection. In your example it is being applied to an Entity. Since an Entity is not a collection, the predicate would not be valid. It would be valid to apply a collection operator to a List property on Entity though.

Something like the following should do what you're after:

let entities = realm.objects(Entity) let id = entities.max("id") as Int? let entity = id != nil ? entities.filter("id == %@", id!).first : nil