Swift diff realm.io 没有提前获取

Question

我想知道是否有可能 realm.io (swift) 到 select 一个 "table" 中不在另一个 "table" 中的所有项目。

假设你有 2 个类:

class A: Object {
    dynamic var id: Int = 0
    dynamic var text: String = ""
}

class B: Object {
    dynamic var id: Int = 0
    dynamic var value: Bool = false
}

是否可以从 A 中获取其 ID 在 B 中不存在的项目的结果？

Answer 1

在 Realm 过滤器 API.

上使用 NSPredicate 实际上有一个非常简单的方法可以做到这一点

func fetch() throws -> [A] {
        do {
            // Create Realm
            let realm = try Realm()

            // Get B objects from Realm and put their IDs to [Int] array
            let IdB: [Int] = realm.objects(B).map { [=10=].id }

            // Create predicate
            // Filter all items where property id is not present in array IdB
            let predicateFilter = NSPredicate(format: "NOT (id IN %@)", IdB)

            // Get all A objects from array using predicateFilter
            let objectsA = realm.objects(A).filter(predicateFilter)

            // Return the [A] array
            return objectsA.map { [=10=] }
        } catch {

            // Throw an error if any
            throw error
        }

}

另请注意，使用 Realm 获取的所有对象都是延迟加载的，这意味着此方法也非常快。来自文档：

All queries (including queries and property access) are lazy in Realm. Data is only read when the properties are accessed.

Swift diff realm.io 没有提前获取

Swift diff realm.io without fetching it in advance

realm

swift

realm-cocoa