NSPredicate Cheatsheet

Common mistakes

Using [NSString stringWithFormat:] to build predicates is prone to have non-escaped diacritics or artifacts like an apostrophe. Use [NSPredicate predicateWithFormat:] instead.

Using OR OR OR instead of IN, results in repeatable code and can be less efficient

When using REGEX and Matches, make sure they are the last part of your predicate statement so it does less work. This way objects will be filtered before doing more heavy look ups.

Using SELF

When using a predicate on an array, SELF refers to each object in the array. Here’s an example: Imagine you are a landlord figuring out which apartments have to pay their water bill. If you have a list of all the city wide apartments that still need to pay called addressesThatOweWaterBill, we can check that against our owned apartments, myApartmentAddresses.

NSPredicate *billingPredicate = [NSPredicate predicateWithFormat: @"SELF IN %@", addressesThatOweWaterBill]

NSArray *myApartmentsThatOweWaterBill = [myApartmentAddresses filteredArrayUsingPredicate:billingPredicate]

LIKE wildcard match with * and ?

* matches 0 or more characters. For example: Let’s say we have an array of names we want to filter

@[@"Sarah", @"Silva", @"silva", @"Silvy", @"Silvia", @"Si*"]

predicateWithFormat: @"SELF == %@", @"Sarah"

Will return “Sarah”

predicateWithFormat: @"SELF LIKE[c] %@", "Si*"

Will return “Silva”, “silva”, “Silvy”, “Silvia”, “Si*”
? matches 1 character only

predicateWithFormat: @"SELF LIKE[c] %@", "Silv?"

Will return “Silva”, “silva”, “Silvy”

Quick tips

CFStringTransform normalizes strings if diacritic insensitive isn’t enough. For example you could turn Japanese characters into a Latin alphabetic representation. It’s extremely powerful with a lot of methods that you can see here: http://nshipster.com/cfstringtransform/

Make sure your columns are indexed to improve performance of using IN operators

[c] case insensitive: lowercase & uppercase values are treated the same

[d] diacritic insensitive: special characters treated as the base character

predicateWithFormat: @"name CONTAINS[c] 'f'"

Keypath collection queries

Keypath collection queries work best when you work with a lot of numbers. Being able to call the min or max, adding things up, and then filtering results are simpler when you only have to append an extra parameter. By having an array of expenses, you can do a quick check on if something is below or above a range of allowed expenses.

[NSPredicate predicateWithFormat: @"expenses.@avg.doubleValue < 200"]

How subqueries work

SUBQUERY(collection, variableName, predicate)

A subquery takes a collection then iterates through each object (as variableName) checking the predicate against that object. It works well if you have a collection (A) objects, and each object has a collection (B) other objects. If you’re trying to filter A based on 2 or more varying attributes of B.

predicateWithFormat: @"SUBQUERY(tasks, $task, $task.completionDate != nil AND $task.user = 'Alex') .@count > 0"

SUBQUERY(…) returns an array. We need to check if its count > 0 to return the true or false value predicate expects.