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我正在尝试为 NSPredicate 编写一个 SUBQUERY。我的问题是我手头没有 SUBQUERY 的谓词部分的查询。有没有办法在 SUBQUERY 中嵌套一个 NSPredicate？

例如，这是我尝试过的：

let ordersPredicate = NSPredicate()//some predicate passed in
//how do I use the ordersPredicate inside the subquery??
let subQueryPredicate = NSPredicate(format: "SUBQUERY(orders, $x,'%@').@count > 0", ordersPredicate.predicateFormat)

更新：我现在必须通过执行第一个谓词并执行 ANY IN 查询来做一个肮脏的解决方法。它的蹩脚:(因为我正在其他地方执行ordersPredicate以及稍后在管道中..

let fetchRequest = NSFetchRequest()//blah
fetchRequest.predicate = orderPredicate
let orders = //execute fetch
let subQueryPredicate = NSPredicate(format: "SUBQUERY(orders, $x, ANY $x in %@).@count > 0", orders)
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2 回答 2

3

感谢@Willeke 的一些建议，我能够想出一个解决方案。

public extension NSPredicate{
    public func stringForSubQuery(prefix:String) -> String{
        var predicateString = ""
        if let predicate = self as? NSCompoundPredicate{
            for (index, subPredicate) in predicate.subpredicates.enumerate(){

                if let subPredicate = subPredicate as? NSComparisonPredicate{
                    predicateString = "\(predicateString) \(prefix)\(subPredicate.predicateFormat)"
                }
                else if let subPredicate = subPredicate as? NSCompoundPredicate{
                    predicateString = "\(predicateString) (\(subPredicate.stringForSubQuery(prefix)))"
                }
                //if its not the last predicate then append the operator string
                if index < predicate.subpredicates.count - 1 {
                    predicateString = "\(predicateString) \(getPredicateOperatorString(predicate.compoundPredicateType))"
                }
            }
        }
        return predicateString
    }
    private func getPredicateOperatorString(predicateType: NSCompoundPredicateType) -> String{
        switch(predicateType){
        case .NotPredicateType: return "!"
        case .AndPredicateType: return "&&"
        case .OrPredicateType: return "||"
        }
    }
}

这是用法

let ordersPredicate = NSPredicate()//some predicate passed in
let ordersPredicateFormat = orderPredicate.stringForSubQuery("$x.")
let subQueryPredicate = NSPredicate(format: "SUBQUERY(orders, $x, \(ordersPredicateFormat)).@count > 0")
于 2016-04-22T15:04:37.530 回答
2

对于将来遇到此问题的任何人，您无需自己构建谓词字符串，只需predicateFormat直接在子查询字符串中使用即可。它已经被正确转义，所以使用字符串插值。

let ordersPredicate = NSPredicate()
// Add it to the query with string interpolation
let subQueryPredicate = NSPredicate(format: "SUBQUERY(orders, $x, \(ordersPredicate.predicateFormat)).@count > 0")

如果您尝试将其作为参数传递，它将尝试再次转义字符串，这可能不起作用。

// Don't do this!
let subQueryPredicate = NSPredicate(format: "SUBQUERY(orders, $x, %@).@count > 0", ordersPredicate.predicateFormat)
于 2020-03-10T14:43:46.197 回答