Filtering data with predicates

To filter data in SwiftData, we use the #Predicate macro.

The same macro is used to filter data loaded with @Query, and also when selecting data to be deleted with delete(model:).

For instance, if we had a database with albums of songs by bands, we could write a query such as this one to filter all the albums of the Beatles:

@Query(filter: #Predicate<Album> { album in
      album.band.name == "The Beatles"
}) var albums: [Album]

To allow the compiler to correctly handle the type information, with #Predicate, you need to identify the data type of the queried information, in our example, Album.

#Predicate is a macro that converts Swift code into a number of PredicateExpression objects. The predicate is checked at compile time for type safety, unlike NSPredicate from Core Data.

At the moment of writing, the process is not perfect, and #Predicate does not support all the features and operations...