ios - Swift 2.1 Core Data - 保存具有一对多关系的数据，但如果已经存在则不添加 isome 数据

Question

我有一个创建新食谱数据的 ViewController。要创建新食谱，用户需要填写标题、配料等并选择其类别。为此，我建立了具有一对多关系的类别和食谱实体。

在创建函数中，我需要进行以下搜索以查看类别实体是否已经具有 selectedCategory 的值。

如果 selectedCategory 已经存在于 Category 实体中，我需要找到它的索引并将该类别分配给 Recipe Entity 但我不知道我需要在这里编写什么代码。

for category in categories {
    if category == selectedCategory {

       /* if selectedCategory already exists in Category entity, 
          find its index and assign that category to Recipe entity.*/

    } else {
       category.name = selectedCategory
       recipe.category = category
    }
}

context.insertObject(recipe)

do {
    try context.save()
} catch {
    print("Could not save recipe")
}

Answer 1

我创建了一个支持 Core Data 的单 View 应用。

我向数据模型添加了两个实体:

CategoryEntity 具有属性“名称”。RecipeEntity 具有属性“title”。

CategoryEntity 与 RecipeEntity 有一个可选的多对一关系，称为“食谱”。它有一个级联删除规则(如果你删除一个类别，所有相关的食谱也会被删除)。

RecipeEntity 与称为“类别”的 CategoryEntity 具有必需的一对一关系。

这些关系彼此相反。

以下代码添加了一个食谱。它首先获取或创建类别，然后将配方分配给它。

import UIKit
import CoreData

class ViewController: UIViewController {

    @IBOutlet weak var category: UITextField!
    @IBOutlet weak var recipeTitle: UITextField!

    @IBAction func createRecipe(sender: AnyObject) {

        // Apple's out of the box Core Data app exposes a managedObjectContext on the appDelegate, so we'll use it here
        if let appDelegate = UIApplication.sharedApplication().delegate as? AppDelegate {

            // we have to have a category name ...
            if let categoryName = category.text {

                // ... and a recipe title
                if let recipeTitle = recipeTitle.text {

                    // get the category to associate this recipe with 
                    let categoryEntity = obtainCategoryEntity(categoryName, context: appDelegate.managedObjectContext)

                    // create the recipe entity
                    let recipeEntity = (NSEntityDescription.insertNewObjectForEntityForName("RecipeEntity", inManagedObjectContext: appDelegate.managedObjectContext) as! RecipeEntity)

                    // assign the recipe to the category 
                    //  note that you set up relationships in Core Data by assigning entities, 
                    //  not indexes or id fields
                    recipeEntity.category = categoryEntity

                    // set the recipe's title
                    recipeEntity.title = recipeTitle

                    // save it!
                    do {
                        try appDelegate.managedObjectContext.save()
                        NSLog("saved context")
                    } catch let error as NSError {
                        NSLog("failed to save context with error \(error)")
                    } catch {
                        fatalError()
                    }
                }
            }
        }
    }

    func obtainCategoryEntity(name: String, context: NSManagedObjectContext) -> CategoryEntity {

        // this function gets or creates a category entity
        let fetchRequest = NSFetchRequest(entityName: "CategoryEntity")
        fetchRequest.predicate = NSPredicate(format: "name == %@", name)

        //find existing category, if it exists
        if let results = (try? context.executeFetchRequest(fetchRequest)) as? [CategoryEntity] {
            if results.count > 0 {
                // we really should only ever have one match, so return it
                return results[0]
            }
        }

        //category did not exist, so create it
        let categoryEntity = NSEntityDescription.insertNewObjectForEntityForName("CategoryEntity", inManagedObjectContext: context) as! CategoryEntity

        // set the category name
        categoryEntity.name = name

        // return it, but don't save it here - let the save later take care of it
        return categoryEntity
    }
}