Where to find the documentation of the SUBQUERY feature of NSPredicate for Core Data?

Question

Where to find the documentation of the SUBQUERY feature of NSPredicate for Core Data?

Answer 1

Good question... it seems like this isn't well documented.

Here is what I found:

• The most documentation can be found at NSExpression(forSubquery:usingIteratorVariable:predicate:) documentation. It contains two examples and the syntax: SUBQUERY(collection_expression, variable_expression, predicate);
• NSExpression briefly mentions Subquery Expressions and the NSSubqueryExpressionType, which is what is used when you specify SUBQUERY in your predicate.
• The most logical place to find the documentation would be the Predicate Programming Guide, but it is just mentioned a few times; in the String Comparisons, and the Reserved Words sections.
• You can try a Google search limited to apple.com, but this only returns 53 results.

Update:

With the addition of App Extensions, Apple has included more SUBQUERY examples since they are required for complex matching logic.

• In the String Comparisons section of the Predicate Programming Guide, it now includes an example of how to match a UTI:

  SUBQUERY (
      extensionItems,
      $extensionItem,
      SUBQUERY (
          $extensionItem.attachments,
          $attachment,
          ANY $attachment.registeredTypeIdentifiers UTI-CONFORMS-TO "com.adobe.pdf"
      ).@count == $extensionItem.attachments.@count
  ).@count == 1
  

• You can find a more complex example in the App Extension Programming Guide > App Extension Essentials > Handling Common Scenarios section:

  SUBQUERY (
      extensionItems,
      $extensionItem,
      SUBQUERY (
          $extensionItem.attachments,
          $attachment,
          ANY $attachment.registeredTypeIdentifiers UTI-CONFORMS-TO "org.appextension.action-one" ||
          ANY $attachment.registeredTypeIdentifiers UTI-CONFORMS-TO "org.appextension.action-two"
      ).@count == $extensionItem.attachments.@count
  ).@count == 1
  

• There's also an NSPredicate Cheatsheet which discusses SUBQUERY in addition to several other NSPredicate features.

---

Essentially each SUBQUERY is equivalent to filter in Swift. And ANY is equivalent to contains.

So taking this example again:

SUBQUERY (
    extensionItems,
    $extensionItem,
    SUBQUERY (
        $extensionItem.attachments,
        $attachment,
        ANY $attachment.registeredTypeIdentifiers UTI-CONFORMS-TO "com.adobe.pdf"
    ).@count == $extensionItem.attachments.@count
).@count == 1

It would be similar to this in Swift:

extensionItems.filter {
  $0.attachments.filter {
    $0.registeredTypeIdentifiers.contains {
      $0.utiConformsTo("com.adobe.pdf")
    }
  }.count == $0.attachments.count
}.count == 1