ios - 如何为 Realm 中的关系属性调用聚合函数？

Question

有没有办法对一对一和一对多关系中的属性使用聚合函数？这是我正在使用的结构的一个简单示例:

class Toy: Object {
    dynamic var purchaseDate: NSDate?
    dynamic var name: String = ""
    let ratings = List<Rating>()
    dynamic var toyOwner: Person?
}

class Rating: Object {
    var stars: Int = 0
    var comments: String?
}

class Person: Object {
    dynamic var name: String = ""
    dynamic var age: Int = 0
}

我想做一些类似的事情:

// Average age of owner of all toys purchased within a date range
let avgAge: Double = realm.objects(Toy)
    .filter("purchaseDate >= %@ && purchaseDate <= %@", startDate, endDate)
    .avg("person.age")

// Sum up all stars given to all toys purchased within a date range
let totalStars: Double = realm.objects(Toy)
    .filter("purchaseDate >= %@ && purchaseDate <= %@", startDate, endDate)
    .sum("sum(ratings.stars)")

有没有办法在 Realm 中执行这些聚合函数？

Answer 1

尽管 Realm 确实有聚合值函数(请参阅 RealmCollectionType 上的 Aggregate Operations 文档)，但这些函数目前不适用于 person.age 等多级键路径。

但是，您可以通过稍微修改您的查询来使用 Realm 完成大部分繁重的工作:

// Average age of owner of all toys purchased within a date range
let ages = realm.objects(Toy)
    .filter("purchaseDate BETWEEN %@", [startDate, endDate])
    .valueForKeyPath("toyOwner.age") as! [Double]
let avgAge = ages.reduce(0, combine: +) / Double(max(ages.count, 1))

// Sum up all stars given to all toys purchased within a date range
let totalStars = realm.objects(Toy)
    .filter("purchaseDate BETWEEN %@", [startDate, endDate])
    .reduce(0) { sum, toy in
        return sum + toy.ratings.sum("stars")
}