在 swift 3 中获取相关表

Posted 2023-03-05

【中文标题】在 swift 3 中获取相关表

【英文标题】：fetch related tables in swift 3

【发布时间】：2016-11-06 19:48:43

【问题描述】：

我正在尝试获取属于某个品牌的所有模型，但是当我尝试获取外键时，我得到的 NSSET 类型具有某种地址结构，其中包含外键，我是坚持这一点，这是我的代码：

class func insertNew (brandDescription: String, inManagedObjectContext context: NSManagedObjectContext) -> Car_Brand? 
        let request: NSFetchRequest<Car_Brand> = Car_Brand.fetchRequest()
        request.predicate = NSPredicate (format: "brand = %@", brandDescription)
        if let brand = (try? context.fetch(request))?.first 
            return brand
         else if let brand = NSEntityDescription.insertNewObject(forEntityName: "Car_Brand", into: context) as? Car_Brand 
            brand.brand = brandDescription
            return brand
        
        return nil
    
class func fetchModel (brandDescription: String, inManagedObjectContext context: NSManagedObjectContext) -> [String]? 
    let Brand = Car_Brand.insertNew(brandDescription: brandDescription, inManagedObjectContext: context)
    let request: NSFetchRequest<Model> = Model.fetchRequest()
    request.predicate = NSPredicate(format: "id_model = %ld", (Brand?.id_model)!)
    request.propertiesToFetch = ["model"]
    request.sortDescriptors = [NSSortDescriptor(key: "model", ascending: true)]
    if let preModelList = try? context.fetch(request) as [Model] 
        var modelList: [String] = []
        for preModelList in preModelList 
            modelList.append(preModelList.model!)
        
        return modelList
     else 
        return nil
    

您能帮我了解如何获取相关表吗？

【参考方案1】：

您的fetchModel 每次调用它时都会插入一个Car_Brand 的新实例。新插入的实例上不太可能存在任何现有模型。

附：它们不是 Core Data 中的“表格”。我们有实体和关系。但是将 Core Data 视为数据库会让您在未来遇到很多逻辑问题。

【讨论】：

谢谢，只有在不存在时才插入，我试图将谓词修改为这样的东西，request.predicate = NSPredicate(format: "Car_Brand.id_model = %@",brandDescription) 但是也不起作用，你知道我在哪里可以找到一些好的例子来过滤这种方式

【参考方案2】：

解决方案：

class func fetchModel (brandDescription: String, inManagedObjectContext context: NSManagedObjectContext) -> [String]? 
        let request: NSFetchRequest<Model> = Model.fetchRequest()
        request.predicate = NSPredicate(format: "id_model.brand = %@",brandDescription)
        request.propertiesToFetch = ["model"]
        request.sortDescriptors = [NSSortDescriptor(key: "model", ascending: true)]
        if let preModelList = try? context.fetch(request) as [Model] 
            var modelList: [String] = []
            for preModelList in preModelList 
                modelList.append(preModelList.model!)
            
            return modelList
         else 
            return nil
        
    

参考： https://developer.apple.com/library/prerelease/content/documentation/Cocoa/Conceptual/Predicates/Articles/pUsing.html