Simple Migration in SwiftData does not work

Question

I have an app with some SwiftData Models. One is called Tapas and it has a relationship with TapasAsana. One Tapas has many Asanas, so I connected them with a relationship. This all works wonderful so far.

Now I just forgot one property in TapasAsana. I read about light migration and it should be easy to just add that property. But no matter if I add it at the end or in the middle of the properties it breaks my app. The app compiles and runs, but the old Data doesn't get shown anymore. And cretaing new ones in the app also won't work, trying to directly create a TapasAsana crashes the App.

This is the code for the TapasAsana Model, the one where I am adding a property. The new property is called recoupCount.

import Foundation
import SwiftData

@Model
class TapasAsana {
    var name: String
    var duration: Int           /// How long to execute daily
    var timeToday: Int          /// Time done already today
    var recoupCount: Int      /// How many recoups this Asana has.
    
    var polar: Bool             /// If it is a polar Asana
    var doneToday: Bool         /// If it is completed today
    
    init(name: String = "Padahastasana", duration: Int = 5 * 60, timeToday: Int = 0, recoupCount: Int = 0, polar: Bool = false, doneToday: Bool = false) {
        self.name = name
        self.duration = duration
        self.timeToday = timeToday
        self.recoupCount = recoupCount
        self.polar = polar
        self.doneToday = doneToday
    }
}

I tried to just add the new Property, to add it at the last position to not confuse SwiftData with the order. But none of that works. I didn't try more because reading about migration said the simple add of a property should work fine.

Joakim Danielson · Accepted Answer · 2024-11-06 17:33:57Z

1

You need to give your new property a default value for a lightweight migration to work and it must be where it's declared, doing so in the init parameter list isn't enough.

@Model
class TapasAsana {
    //...
    var recoupCount: Int = 0
    
    //...
}

answered Nov 6, 2024 at 17:33

Joakim Danielson

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6 Comments

Sphere Over a year ago

Thank you so much! I thought exactly as you assumed, that the init default would be the one asked for the migration. Could you shortly explain why SwiftData needs such explicit declaration for the Migration to work?

Joakim Danielson Over a year ago

When you add a new property that is not optional then it must have a value in the underlying storage (database) so we need to supply it.

Navin Samuel Feb 26 at 3:11

@JoakimDanielson Also, we can add new nullable property right (but not setting nil value). This is eligible for automatic lightweight migration right ? i.e. var employeeName: String?

Joakim Danielson Feb 26 at 7:45

@NavinSamuel yes and yes, optional properties only need a lightweight migration and yo don’t need to supply a nil value.

Joakim Danielson Feb 26 at 12:41

You should post a new question for that.

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