Quick Explanation of SUBQUERY in NSPredicate Expression

Question

There appears to be zero documentation about the SUBQUERY keyword from Apple and I can\'t find a simple explanation about it on SO or on Google. It\'s a conspiracy! ;)

Answer 1

Subquery represents a predicate (third argument - $x IN %@) that is evaluated on all objects (second argument - $x - it's like a variable name in foreach) of a relationship (first argument - Bs). Similarly to regular query returns a list of objects.

I see in many places that people use $x almost dogmatically, but $object in objects relationship makes perfect sense as well (or $city in cities...) :)

I've written a blog post about SUBQUERY some time ago. You can check it here.