Answer from Perplexity:
To fix this issue, replace unsupported types with alternatives that SwiftData can handle:
For Duration: Use TimeInterval instead, as it is supported by SwiftData.
For Color.Resolved: Avoid using it directly in your models. Instead, store color-related data in a format like RGB values or another supported type.
This ensures compatibility with SwiftData's encoding requirements.
Heeeelp!
What does the following error mean, and how do I fix it? It happens, when I call `modelContext.save()` in a SwiftUI view.
"SwiftData/ModelCoders.swift:105: Fatal error: Composite Coder only supports Keyed Container”
Edit: Solved! It appears SwiftData does not support Duration properties in models. Changing it to a TimeInterval instead fixes it.
Biggest tip I can offer when working through a migration strategy, is to work on a new git branch so you can delete a bad data store, restore the old working branch and start over.
You can create a rollback strategy as well but I found it just as easy to delete the bad data store, revert to working data and try again.
If you don't start a migration clean and it has previously failed you may find a lot of duplicate data once it does work all the way thru.
In my case I was migrating from a schema where a relationship was becoming many to many. I needed to gather all the current single relationships in willMigrate and then in didMigrate I had to add them to the new to many relationship.
There is no relationship between willMigrate and didMigrate so you need to do some sketchy (globals) things to pass data across contexts.
Once I'm comfortable I've gotten this all correct, I’ll do a blog post.
You do not need to update any models in willMigrate if they are the same across schemas.
You may only need to make changes in didMigrate, for example setting defaults for new Model values.
TIL working with #SwiftData migrations the past few days.
The documentation is non-existent. The sample code, tutorials, etc are all copying mostly trivial examples and sadly each other.
A custom migration schema has two phases each is passed a context. The first, willMigrate, will have the context for the schema you are migrating from, the second, didMigrate will have the newest context.
Learn how to write flexible & efficient dynamic queries using SwiftData — perfect for building scalable SwiftUI apps. 
Watch now 
Learn how to load local JSON, decode it into SwiftData @Model objects, and display it using SwiftUI’s List view! 
SwiftData Architecture - Patterns and Practices
https://azamsharp.com/2025/03/28/swiftdata-architecture-patterns-and-practices.html
Sorry @mattiem for giving yet another example of strong behavior of ModelActor. I hope maybe you can understand why it’s behaving so strangely. 
#SwiftData #ModelActor
S4E5: @mikaelacaron and @twostraws talk networking – mostly about getting computers to talk to each other, but also, just as dauntingly, about talking to other humans at conferences too. 
Plus: news, Apple delays, and the trifling little matter of the major changes you’re hoping to see in #Xcode, #SwiftUI, #SwiftData and more as #WWDC25 rolls around!
Took a deep dive into SwiftData today! 
Took a deep dive into SwiftData today!
A few months ago, I wrote a comprehensive course on SwiftData. Its just been published for Kodeco subscribers.
https://www.kodeco.com/ios/paths/continuing-swiftui/45123174-data-persistence-with-swiftdata
Build a Gardening App with SwiftData & SwiftUI 
This morning's struggle with the "magical" framework:
https://stackoverflow.com/questions/79512672/using-swiftdatas-query-makes-the-app-unresponsive
How come #SwiftData `@Query` predicates can't rely on external values? How can that make sense??
Source: @twostraws excellent overview of queries:
https://www.hackingwithswift.com/quick-start/swiftdata/how-to-filter-swiftdata-results-with-predicates
Enjoy and share :)
Watch it here:
https://www.youtube.com/playlist?list=PLDMXqpbtInQi5WU_YbOyef7WRcKj_0SO-
13 Build a Gardening App with SwiftUI & SwiftData | iOS Development Tutorial | Part 13
In this video, Mohammad Azam adds validation to AddNoteScreen & refactors NoteListScreen by extracting a reusable NoteCellView for a cleaner UI.
