Predicate for comparing the month component of a Core Data date attribute

Question

I have a Core Data entity which has a date attribute. I would like to write a predicate to extract all dates within a specific month, e.g. July, irrespective of year. How can this be achieved? Thanks

Answer 1

You can create a new method on your entity, which we'll call monthOfDate. This method simply uses [self dateAttribute] and NSDateComponents to extract what the month of the date is.

Then you can write a predicate that does:

//assuming 1-based month indexing
NSPredicate * julyEntities = [NSPredicate predicateWithFormat:@"monthOfDate = 7"]; 

The trick here is realizing that the left keypath of your predicate will result in a method invocation. So if you set the left keypath to "monthOfDate", it will end up invoking your monthOfDate method and using the return value of the method as the comparison value in the predicate. Neat!