Core Data and uniquing?

Question

Either I'm not understanding what the term "uniquing" means in Core Data, or I'm not fetching my data properly. I have a pretty simple data model. Three entities: Community, Asset, and Category. Each Community has a relationship to multiple categories. Each category has a relationship to multiple assets. Each asset that is created must have one and only one category.
In the code I've posted, I'd like to output all the categories that a specific community has into the console. I thought that because of Core Data's uniquing capability, only one category of the same name could exist at a time (name is the only attribute for a category). However, when I print to the console, I'm getting duplicate category names.
// 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);
When I execute this, I get duplicate names in the console. Why?
Thanks for the help!

Answer 1

You need to set the name as a key or specify unique when creating the table and that column. Generally it is bad to use a city name as a unique id, since there are many cities with the same name (Middletown) in different states. You should use a number, autoincrement, as the id.