SwiftData-OrderedRelationship

A Swift macro taking away the pain in adding ordered relationships to SwiftData models.

Description

Many SwiftData projects require explicitly ordered relationships. SwiftData-OrderedRelationship is a Swift macro that takes away the pain in implementing this and has implicit CloudKit Sync Conflict Resolution. No special handling on your part is required, when an array with a new order is supplied, OrderedRelationship does all the work for you.

CloudKit Sync

OrderedRelationship is designed to support CloudKit synchronization, providing fast syncing and implicit conflict resolution.

Fast Syncing

OrderedRelationship doesn't store the order of its elements as the index of each element. It rather stores each elements position as a random number between Int.min and Int.max. When B gets inserted between A and C with the positions 100 and 104[^1], the position of B will be randomly chosen between 101, 102 and 103. This means only one object needs to be synced per change.

Conflict Resolution

As detailed in fast syncing, elements do not have consecutive position numbers, but are distributed randomly between Int.min and Int.max. By randomly choosing a new number between the numbers inbetween which a new element is inserted or an existing one is moved, having the same operation take place on two different devices with detached state does not require any explicit conflict resolution or re-assigning of indices after both changes have synced to either device.

Example

Say you have a SubItem model:

@Model
class SubItem {
    init() {}
}

Then you can add an ordered relationship to its container Item:

@Model
final class Item {
    @OrderedRelationship
    var orderedSubItems: [OrderedSubItem]? = []
    
    init() {}
}

The type of the variable is an optional array of an undefined type, preferably the type you want to be ordered with a single word prefix. This type (OrderedSubItem in this example) will be defined by the @OrderedRelationship macro.

Now you add the inverse relationship to the SubItem model:

var superitem: Item.OrderedSubItem? = nil

That's it. There is nothing more you need to do.

Resulting Code

The resulting code will contain not only the OrderedSubItem model, but also a new variable:

var subItems: [SubItem]

The variable name is inferred from your custom variable name, removing the first word. You can also specify it explicitly using the arrayVariableName argument. You can both get and set this array. All the work of storing the new order will be performed for you.

Arguments

The @OrderedRelationship macro supports arguments to customize its behavior. All of them are optional, as seen in the example above.

containingClassName

The name of the class containing the declaration (Item in the example above). If nil, this will be inferred by the name of the file in which the macro resides.

itemClassName

The name of the class that the items are. If nil, will be inferred by name of the declared array contents without the prefix. In the example above with the array contents being of type OrderedSubItem, the type should be SubItem.

inverseRelationshipName

The name of the property in itemClassName to use for the inverse relationship. Defaults to superitem, and in the example above would be SubItem.superitem. You are always required to create a property for the inverse relationship, and this argument allows you to tell the macro what it is.

arrayVariableName

The variable name of the resulting array. If nil, will be inferred by the name of the declared variable without the prefix. In the example above with the variable name being called orderedSubItems, the resulting array will be called subItems.

deleteRule

The delete rule to apply to the items. The default value is .cascade.

Why is the macro applied to the ordered items and not the resulting array?

You might wonder why the @OrderedRelationship macro is applied to the stored array and not the resulting array, like this:

@Model
final class Item {
    @OrderedRelationship
    var subItems: [SubItem]
    
    init() {}
}

While this code is easier to understand, it is simply an impossibility to create such a macro. Macros are expanded alongside each other on a code level basis. This means that the first macro to be expanded in the above code would be the @Model macro. The @Model macro cannot see the results of the @OrderedRelationship macro, since it is expanded after the @Model macro. That is why the stored array has to be the one that is declared by the macro user.

[^1]: The positions being this close to each other is a statistically irrelevant event, this is just an example to showcase the method.