I recently implemented an ordered, reflexive relationship in core data - similar to the many-to-many Friend example in the Core Data Programming Guide. In that example there is the main Person table, and a join entity called FriendInfo which also has a ranking attribute.

In my use case the join entity has a 'type' and I needed Core Data to efficiently select related entities by type and order. To explore the performance and see what core data actually does I created a small model and tried several approaches to fetching, using SQL debug logging to dump the queries. I ran a release build on my iPhone 3G device to capture the logging and aggregated logs into a spreadsheet to summarize run times.

This article summarizes the general problem of programmatically selecting a region from an iPhone image that may be rotated (i.e. has an embedded EXIF orientation tag) and my solution.

Some Objective-C iPhone code I've been writing recently needed to extract a region of the original UIImage provided to UIImagePickerControllerDelegates. If you've used this controller you'll know that when using a UIImagePickerController to select an image source, the full original image and optionally a selected region is returned (assuming the user didn't cancel). The nice thing about the selected region is that it is already oriented vertically in relation to how it appears on-screen.

In contrast, the full image is not rotated but still contains EXIF rotation data, which is made available to your code via the UIImage.orientation property. This is where you have to manage your selection rects properly.