iPhone SDK：核心數據和uniquing？

Question

要麼我不理解Core Data中術語「uniquing」的含義，要麼我沒有正確讀取數據。我有一個非常簡單的數據模型。三個實體：社區，資產和類別。每個社區都與多個類別有關係。每個類別都與多個資產有關係。每個創建的資產必須只有一個類別。

在我發佈的代碼中，我想輸出特定社區所有類別的控件。我認爲，由於Core Data的獨特功能，每次只能存在一個同名的類別（名稱是類別的唯一屬性）。但是，當我打印到控制檯時，我收到重複的類別名稱。

// Fetch Community instances in the database, and add them to an NSMutableArray 
NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
NSEntityDescription *community = [NSEntityDescription entityForName:@"Community" inManagedObjectContext:managedObjectContext]; 
[request setEntity:community]; 

// Only return the community instances that have the cityName of the cell tapped in the CommunitiesNonEditableTableViewController 
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(cityName like %@)", cellCityName]; 
[request setPredicate:predicate];  

NSError *error; 
NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy]; 

if (mutableFetchResults == nil) { 
    // Handle the error. 
} 

// Set communitiesArray with mutableFetchResults 
[self setCommunitiesArray:mutableFetchResults]; 

[mutableFetchResults release]; 
[request release]; 

// Creates a community instance using the community stored in the array at index 0. This is the only community in the array. 
Community *communityInstance; 
communityInstance = [communitiesArray objectAtIndex:0]; 

// Retrieves existing categories of assets in the community, and adds them to an NSSet 
NSSet *communityCategoriesSet = communityInstance.categories; 

// Converts NSSet to an NSArray with each category as an index 
NSArray *communityCategoriesArray = [communityCategoriesSet allObjects]; 

// For loop that iterates through the array full of categories, retrieves the names of each category, and adds it to an NSMutableArray 
categoryNames = [[NSMutableArray alloc] init]; 
int i; 
for (i = 0; i < [communityCategoriesArray count]; i++) { 
    Category *categoryInstance; 
    categoryInstance = [communityCategoriesArray objectAtIndex:i]; 
    [categoryNames addObject:categoryInstance.name]; 
} 

// Prints array full of category names to console 
NSLog(@"%@", categoryNames); 

當我執行此操作時，我在控制檯中得到重複的名稱。爲什麼？

Answer 1

抽樣意味着對象圖中的每個對象都是唯一的。這並不意味着任何兩個對象的屬性都不相同。非抽象是關於關係而不是屬性。沒有兩個對象可以在對象圖中佔據完全相同的位置。

至於爲什麼你在輸出中得到多個類別：最簡單的解釋是communityInstance.categories是一對多的關係。 （因爲它有一個複數名稱並將其分配給集合。）在一對多關係中，上下文不強制關係另一端的單個對象。