Database Reference
In-Depth Information
Adding managed object subclasses
In the previous chapter you learned how to create custom managed object
subclasses for your Core Data entities. It's more convenient to work this way, so
this is what you'll do for
Dog
and
Walk
as well.
Go to
Editor\Create NSManagedObject Subclass…
and choose the
Dog Walk
model and then both the
Dog
and
Walk
entities. Make sure you choose
Swift
as
the language and click
Create
.
Dog.swift
should look like this:
import
Foundation
import
CoreData
class
Dog:
NSManagedObject
{
@NSManaged
var
name:
String
@NSManaged
var
walks:
NSOrderedSet
}
Like before, the
name
attribute is a
String
. But what about the
walks
relationship?
Core Data represents to-many relationships using sets, not arrays. Because you
made the
walks
relationship ordered, you've got an ordered set.
Note:
NSSet
seems like an odd choice, doesn't it? Unlike arrays, sets don't
allow accessing their members by index. In fact, there's no ordering at all!
Core Data uses
NSSet
because a set forces uniqueness in its members. The
same object can't feature more than once in a to-many relationship.
If you need to access individual objects by index, you can check the
Ordered
checkbox in the visual editor, as you've done here. Core Data will then
represent the relationship as an
NSOrderedSet
.
Walk.swift
should look like this:
import
Foundation
import
CoreData
class
Walk:
NSManagedObject
{
@NSManaged
var
date:
NSDate
@NSManaged
var
dog:
Dog
}
The inverse relationship back to
Dog
is simply a variable of type
Dog
. Easy as pie.
