I have a core data-based app that manages records of auto dealerships. Each record stores the dealer's address, which is broken into
zip components, each stored as a string in the data store.
I would like to present a list of cities with dealerships to the user, so I'm trying to figure out if it is possible to get a list of every unique city name that has been entered into the store. I other words, is it possible to issue some sort of query against all of the dealership records that will return a list of distinct city names?
I would know how to do this easily with a SQL query, but (how) is this done in Core Data?
Thanks very much!
Core Data have the option to get distinct record. The method of getting unique results using NSArray and NSSets are not recommend.
NSDictionary *entityProperties = [entity propertiesByName];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:[entityProperties objectForKey:@"<<yourattrib>"]]];
Refer Apple documentation and check the answers for How to fetch distinct values in Core Data?
You're right, there isn't an "easy" way to do this with Core Data, because Core Data is not a database. However, it is possible. Here's the general idea:
- Fetch all your Dealer objects via an
NSFetchRequest. To simplify the query, you can set the fetch request to only fetch the
NSArray * uniqueCities = [fetchedDealers valueForKeyPath:@"@distinctUnionOfObjects.city"];
A quick way to ensure a unique set of things is to use NSSet. Once you have the results for a query on city take your NSArray and do
NSSet* uniqueResults = [NSSet setWithArray:resultsArray];
You can transform the set into another collection class if more convenient or just the object enumerator to do something with all of them. I do not know if this approach or the valueForKeyPath method from Dave DeLong is more efficient.