核心數據謂詞對許多

Question

我有一個核心數據模型，有一對多的關係，有一個類別，它可以包含許多子類別。

類別< ---- >>子目錄

我試圖執行一個取來檢查，如果一個特定的類別中包含有特定名稱的子目錄。假設我有以下兩個類別，我想要查看名爲「Fruits」的類別中是否有任何子類別名稱爲「Apple」。

Vetegables 
    - Carrot 
    - Lettuce 

Fruits 
    - Apple 
    - Orange 
    - Pear 

代碼：

- (SubCategory *)searchForSubCategoryWithName:(NSString *)subCategory 
            inCategory:(Category *)category 
{ 
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; 
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"SubCategory" inManagedObjectContext:self.beer.managedObjectContext]; 
    [fetchRequest setEntity:entity]; 

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == [c] %@", subCategory]; 
    [fetchRequest setPredicate:predicate]; 

    NSError *error; 
    NSArray *fetchedObjects = [self.beer.managedObjectContext executeFetchRequest:fetchRequest error:&error]; 

    if (fetchedObjects != nil && fetchedObjects.count > 0) { 
     return [fetchedObjects objectAtIndex:0]; 
    } 
    else { 
     return nil; 
    } 
} 

Answer 1

儘量尋找其中有你喜歡的不是其他方式，然後從返回結果category.subcategory子類別類別的實體。我不知道你的NSManagedObjects看怎麼樣，但試試這個：

- (SubCategory *)searchForSubCategoryWithName:(NSString *)subCategory 
            inCategory:(Category *)category 
{ 
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; 
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Category" inManagedObjectContext:self.beer.managedObjectContext]; 
    [fetchRequest setEntity:entity]; 

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"subcategory.name == [c] %@ AND name == %@", subCategory, category.name]; 
    [fetchRequest setPredicate:predicate]; 

    NSError *error; 
    NSArray *fetchedObjects = [self.beer.managedObjectContext executeFetchRequest:fetchRequest error:&error]; 

    if (fetchedObjects != nil && fetchedObjects.count > 0) { 
     Category *categoryManagedObject = [fetchedObjects objectAtIndex:0]; 
     return categoryManagedObject.subcategory; 
    } 
    else { 
     return nil; 
    } 
} 

...或者只是添加類別到您的謂詞，你應該在你的子類別管理對象定義：

- (SubCategory *)searchForSubCategoryWithName:(NSString *)subCategory 
             inCategory:(Category *)category 
    { 
     NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; 
     NSEntityDescription *entity = [NSEntityDescription entityForName:@"Category" inManagedObjectContext:self.beer.managedObjectContext]; 
     [fetchRequest setEntity:entity]; 

     NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == [c] %@ AND category == %@", subCategory, category]; 
     [fetchRequest setPredicate:predicate]; 

     NSError *error; 
     NSArray *fetchedObjects = [self.beer.managedObjectContext executeFetchRequest:fetchRequest error:&error]; 

     if (fetchedObjects != nil && fetchedObjects.count > 0) { 
      return [fetchedObjects objectAtIndex:0]; 
     } 
     else { 
      return nil; 
     } 
    }