NSPredicate with dynamic key and value

Question

I am trying to create a function that searches through my core data stack and return the amount of messages for a desired key/value.

Here is my code:

Answer 1

 let predicate = NSPredicate(format: "SELF[%@] CONTAINS %@",key,value)

This worked for me

Answer 2

To substitute a key in the predicate string, you have to use %K, not %@:

[NSPredicate predicateWithFormat:@"%K == %@", key, value];

From the Predicate Format String Syntax documentation:

• %@ is a var arg substitution for an object value—often a string, number, or date.
• %K is a var arg substitution for a key path.