问题
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!
回答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}"];
回答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).
回答3:
I think that it's taking text.lengh as a relationship. Try to set a predicate only with position and then do a loop looking for text.length == 5.
来源：https://stackoverflow.com/questions/19889211/coredata-predicate-string-property-length