Core Data Fetch

Question

I have an entity, and I want to fetch a certain attribute.

For example,

Let's say I have an entity called Food, with multiple attributes. I want to select all categories, which is an attribute on each food item. What's the best way to accomplish this in Core Data?

Answer 1

You can only fetch whole objects, but you can only fetch objects that have a perticlar attribute using nspredicate in your fetch request. See the code snippet in Xcode in the snippet section. Look for fetch request in the find field and drag out the snippet with the nspredicate code. You can set the predicate to only find the objects that satisfy this predicate. Hope this helps!

Answer 2

EDIT: I think I misunderstood your question. It seems that you only want to fetch a property of an object, not the object itself (e.g. the attribute but not the entity)? I don't believe core data is going to work that way...it's an object graph rather than a database, as the person above mentioned. Research how Core Data "faults", automatically retrieving dependent objects as they are needed. I left the advice below in case it still applies, though I'm not sure it will.

You can add a predicate to your search in order to fetch only objects which meet certain criteria; it works like an "if" statement for fetching. Here's Apple's documentation:

https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSPredicate_Class/Reference/NSPredicate.html

And a tutorial: http://www.peterfriese.de/using-nspredicate-to-filter-data/

All that said, the necessity really depends on how many objects you're fetching. If it's not causing any performance hit, there's not necessarily anything wrong with fetching a few un-needed objects. Solve problems, in other words--don't "optimize" things that were working fine. If your model contains a ton of objects, though, it could be expensive to fetch them all and you would want to use a predicate.

Answer 3

Just run your fetch request and then use valueForKey: to extract all of the attribute values. If your model contains lots of objects, you can set the fetch limit and offset (and sort descriptor) to page through the items. When doing this you should also set the fetch request to not return objects as faults.

Just remembered there is an alternative. You can set the properties to fetch and then set the result type to NSDictionaryResultType. You still need to do the iteration but this will return the minimum data possible.