ios - 是否可以创建 NSFetchedResultsController 来获取特定属性具有不同值的所有实体？

Question

假设我有 Person 实体，其属性:id、name、age:

我有以下记录:

101, Katherine, 23
102, Madelaine, 23
103, Katherine, 27
104, Vicky, 18
105, Kirsten, 45
106, Vicky, 12

And I need all records sorted by age, without duplicating name.

The output should be:

105, Kirsten, 45
103, Katherine, 27
102, Madelaine, 23
104, Vicky, 18

This is how I create NSFRC:

    let context = NSManagedObjectContext.mr_default()
    let fetchRequest = NSFetchRequest<Person>(entityName: "Person")
    let ageDescriptor = NSSortDescriptor(key: "age", ascending: false)

    fetchRequest.sortDescriptors = [ageDescriptor]

    fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
    fetchedResultsController.delegate = self

如何应用规则来获取所有人员和仅一个(最年长的)同名人员？

Answer 1

您可以使用 NSFetchrequest 中的属性 returnsDistinctResults 和 propertiesToFetch 仅返回提取请求的不同结果。在您的情况下，propertiesToFetch 将是实体的名称。

您可以在此处找到更多信息: https://developer.apple.com/documentation/coredata/nsfetchrequest

就像示例代码一样。 (我是凭自己的想法做的，所以准确的语法可能不是 100% 准确):

let entity = NSEntityDescription.entity(forEntityName: "Person", in: context)

let entityProperties = entity.propertiesByName
let namePropertyDescription = entityProperties["name"]

fetchrequest.propertiesToFetch = [namePropertyDescription]
fetchrequest.returnsDistinctResults = true