CoreData predicate: string property length?

Question

Say if I have an entity Fragment, it has an attribute 'text' which is a string, I want to query the list of Fragment whose text is of length 5:

[NSPredicate predicateWithFormat:@"position == %@ AND text.length == %d", pos, 5];

It does not work (ie returns no result), but if I remove text.length in the query it works and I'm certain that there are texts of length 5, so what do I need to change it to?

Thanks!

Answer 1

There is no length attribute for strings in the NSPredicate. Use regex instead. Your predicate should look as follows:

[NSPredicate predicateWithFormat:@"position == %@ AND text MATCHES %@", pos, @".{5}"];

Answer 2

You cannot use Objective-C functions like length in a Core Data fetch request. But you can replace it with the "LIKE" operator, which does a simple pattern matching:

[NSPredicate predicateWithFormat:@"text LIKE %@", @"?????"];

An interesting point is that Core Data does not throw an exception or return with an error, but just ignores the length method, i.e. it just uses the predicate "text = '5' instead. This can be seen by activating Core Data debug output by setting the launch argument

-com.apple.CoreData.SQLDebug 3

(which is generally a good method to locate Core Data fetch problems).