预加载具有多对多关系的核心数据的最佳实践

2 回答

• 1

  我将创建三个实体，Supermarket，Product和SupermaketProductDetails，其属性和关系如下：

  Supermarket:
  Attributes: name, location, phone number
  Relationship (to many): productDetails
  
  Product:
  Attributes: name, price, company
  Relationship (to many): supermarketDetails
  
  SupermarketProductDetails:
  Attributes:  priceofProductforGivenMarket, primaryProductforGivenMarket
  Relationships: (to one) supermarket, (to one) product
  

  在Xcode中，指示超市实体中productDetails关系的“目的地”是SupermarketProductDetails实体，使用逆超市，同样将SupermarketProductDetails实体设置为Product实体中的supermarketDetails的目的地，具有逆产品 .

  然后我会首先解析超市的JSON数据，创建超市对象并设置名称，位置和电话号码，但不输入产品关系的任何内容 . 同样，解析产品JSON并创建所有Products对象 . 然后我将解析连接表，并创建SupermarketProductDetails对象 . 根据您的JSON数据设置属性，并执行获取以获取具有正确名称的超市实体，同样获取具有正确名称的Product实体，然后直接设置这些关系 .

  EDIT: 假设您将连接表中的每一行解析为四个NSStrings：nameofsupermarket，nameofproduct，priceofProductforGivenMarket和primaryProductforGivenMarket . 那么对于每一行......

  // First fetch the correct Supermarket...
  NSFetchRequest *supermarketFetch = [NSFetchRequest fetchRequestWithEntityName:@"Supermarket"]
  NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name like %@",nameofsupermarket];
  supermarketFetch.predicate = predicate;
  NSError *error;
  NSArray *results = [context executeFetchRequest:supermarketFetch error:&error];
  // should really check for errors, and that we get one and only one supermarket
  NSLog(@"Supermarket fetch returned %i results",[results count]);  // should be only 1!
  mySupermarket = (NSManagedObject *)[results firstObject];
  if (![[mySupermarket valueForKey:@"name"] isEqualToString:nameofsupermarket]) {
      NSLog(@"Wrong supermarket fetched");
      }
  
  // Now fetch the product...
  NSFetchRequest *productFetch = [NSFetchRequest fetchRequestWithEntityName:@"Product"]
  predicate = [NSPredicate predicateWithFormat:@"name like %@",nameofproduct];
  productFetch.predicate = predicate;
  results = [context executeFetchRequest:productFetch error:&error];
  // should really check for errors, and that we get one and only one product
  NSLog(@"Product fetch returned %i results",[results count]);  // should be only 1!
  myProduct = (NSManagedObject *)[results firstObject];
  if (![[myProduct valueForKey:@"name"] isEqualToString:nameofproduct]) {
      NSLog(@"Wrong product fetched");
      }
  
  // Now create the SupermarketProductDetails entity:
  NSManagedObject *mySupermarketProductDetails = [NSEntityDescription insertNewObjectForEntityForName:@"SupermarketProductDetails" inManagedObjectContext:context];
  // set attributes...
  [mySupermarketProductDetails setValue:primaryProductForGivenMarket forKey:@"primaryProductForGivenMarket"];
  [mySupermarketProductDetails setValue:priceOfProductForGivenMarket forKey:@"priceOfProductForGivenMarket"];
  // set relationships...
  [mySupermarketProductDetails setValue:mySupermarket forKey:@"supermarket"];
  [mySupermarketProductDetails setValue:myProduct forKey:@"product"];
  [context save:&error];
  // should check for errors...
  

  请注意，您只需要设置这些关系的一面--CoreData为您更新另一面（即它将mySupermarketDetails添加到myProduct的supermarketDetails集等） . 还要注意，（一对一）关系（例如超市）的“ Value ”是目标对象本身（例如mySupermarket）;你不要使用名称或任何其他密钥 . Coredata（隐藏在底层的sql表中）使用唯一的objectID来进行链接 .

  （这可能是更有效的方法，而不是为每个SupermarketProductDetails条目进行两次提取，但这将有效 . ）

  EDIT2：请注意，上面假设您的实体都是作为NSManagedObjects实现的 . 如果您为每个实体创建了单独的子类，那么您可以简化上面的一些代码 . 例如，valueForKey：和setValue：forKey：可以使用点表示法替换为等效的属性访问器方法，例如：

  [mySupermarketProductDetails setValue:primaryProductForGivenMarket forKey:@"primaryProductForGivenMarket"];
  

  会成为：

  mySupermarketProductDetails.primaryProductForGivenMarket = primaryProductForGivenMarket;
  

  和

  [mySupermarket valueForKey:@"name"]
  

  可以替换为：

  mySupermarket.name
  

  同样，应使用适当的子类而不是NSManagedObject创建对象 . 例如 .

  NSManagedObject *mySupermarketProductDetails = [NSEntityDescription insertNewObjectForEntityForName:@"SupermarketProductDetails" inManagedObjectContext:context];
  

  会成为

  SupermarketProductDetails *mySupermarketProductDetails = [NSEntityDescription insertNewObjectForEntityForName:@"SupermarketProductDetails" inManagedObjectContext:context];
  

  回复于 2024-04-25T00:53:24+08:00
• 1

  我认为上面的方法很难预取，并且会预取几乎所有匹配的条目，并且预取的真正动机将会消失 .

  我还建议在超市表中添加topProduct .

  您必须在两个方向上定义多对多关系 - 也就是说，您必须指定两个关系，每个关系都是另一个关系的倒数

  Edit 01:

  Product (Attributes):
  name
  price
  company
  
  Supermarket (Attributes):
  name
  location
  phone number
  supermarket top Product
  

  一旦你创建了类似上面的属性并添加了多对多的关系 .

  Coredata将在下面添加 relationships 的相应实体 .

  **NSSet products, [supermarket table] and NSSet supermarkets [product table]**
  

  所以现在你可以在插入数据之后实际更新这种关系：即

  Insert

  // Create Product
      NSManagedObject *product = [[NSManagedObject alloc] initWithEntity:@"Product" insertIntoManagedObjectContext:self.managedObjectContext];
  
      // Set details
      [product setValue:@"xxx" forKey:@"name"];
      ...
  
  // Create Supermarket
  
      NSManagedObject *supermarket = [[NSManagedObject alloc] initWithEntity:@"SuperMarket" insertIntoManagedObjectContext:self.managedObjectContext];
  
      // Set First and Last Name
      [supermarket setValue:@"Main Street" forKey:@"name"];
      ...
  
  // Create Relationship
  
      [supermarket setValue:[NSSet setWithObject:product] forKey:@"products"];
  
      // Save Managed Object Context
      NSError *error = nil;
      if (![supermarket.managedObjectContext save:&error]) {
          NSLog(@"Unable to save managed object context.");
          NSLog(@"%@, %@", error, error.localizedDescription);
      }
  
  //And similarly inverse relationship.
  

  Fetching

  此外，由于您在超市表中有关于产品的相关信息，反之亦然，因此提取将是平滑的之前讨论过，不会预先预取所有数据，否则它会做 . 因为没有必要通过coreData fetch创建进一步的关系 .

  Please follow for more details: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html#//apple_ref/doc/uid/TP40001857-SW10

  另请注意：Core Data sectionNameKeyPath with Relationship Attribute Performance Issue

  回复于 2024-04-25T00:53:24+08:00