Enhance my Core Data design. Experts only!

Question

In AcaniUsers, I'm downloading the closest 20 users to me and displaying their profile pictures as thumbnails in a table view. User & Photo are both Resources because they each have an id (MongoDB BSON ObjectId) on the server. Each user has a unique_id. Each Photo has four different sizes (images) on the server: square: 75x75, square@2x: 150x150, large: 320x480, large@2x: 640x960. But, each device will only have two of these sizes, depending on whether it's an iPhone 3 or 4 (retina display). Each of these sizes has their own MongoDB collection. And, all four images for each Photo have the same BSON ObjectId's across these four collections.

In the future, I may give User a relationship called photos to allow a user to have more than one photo. Also, although I don't foresee this, I may add more Image sizes (types).

The fresh attribute on Image tells me whether I've downloaded the latest Image. I set this to NO whenever the Photo's ID has changed, and then back to yes after I've finished downloading the Image.

1. Should I store the four different images in Core Data or on the file system and just store their URLs in Core Data? I read somewhere that over 1 or 2MB, you should store in file system, not Core Data. So, I was thinking of storing the square images in Core Data and the large images in the file system, but I'd rather store them all the same way to make things easier. So, maybe I'll just store them all in the file system? What do you think?

2. Do you think I should discard the 75x75 & 320x480 sizes since pretty soon iPhone 3's will be gone?

3. How can I improve my design of the entities, and their attributes and relationships. For example, is the Resource entity even beneficial at all?

4. I'm displaying the Users with an NSFetchedResultsController. However, it doesn't know when the User's image gets updated, so the images don't show up until I scroll aggressively the first time. How do I let the NSFetchedResultsController know that a user's thumbnail has finished downloading? Do I have to use KVO?

Answer 1

To answer your questions:

1 I'd store them all in the file system and record the URL in the database. I've never been a big fan of storing image data in the DB. Plus it'll simplify things a little to have all of the image storage uniform. That way in your image loading code you don't have to worry about if it's a type that's stored in the DB or on the file system.

2 No, I wouldn't do that yet. The iPhone 3 is going to be around for a bit longer. ATT is still selling them as the cheap entry level iPhone. I just saw a commercial the other night advertising them for $49.

3 Remove the Resources entry and add the id attribute to each of the classes. How you did it is actually bad. Abstract entities should only be used when you have a couple of entities that are almost identical and only have a few differences between them. Under the hood, Core Data will make only one table for an abstract entity and all of its children. So right now you're going to end up with only one table that will contain both your user and photo entries which can be bad when you're trying to query just type of entity.

You should also delete the Image entity and move its attributes into the Photo entity. The Photo will always have those values associated with it and the same values won't be shared between photos. Having them as a separate entity will cause a slow down. You'll either need to load them with the photos which will require a join (slow) or they'll be loaded one at a time when you access either the data or fresh attributes which is also slow. When each of the faults is fired in the latter scenario a separate query and round trip to the disk will happen for each object. So when you loop through your pictures for display in the table, you'll be firing n queries instead of one which can be a big difference in performance.

4 You can use KVO to do it. Have your table cell observer the User or Picture (depends on if you have the Picture already added to the user and are changing the data or if you're adding a new picture to the user on load completion). When the observer gets triggered, update the image being displayed.