TL;DR: When using CloudKit with Core Data or SwiftData, avoid unique constraints, undefined types, and non-optional attributes without defaults. Relationships must be optional and have inverse settings. Use lightweight migrations for adjustments, adhering to “add-only” changes.
Background
When using Core Data or SwiftData for cloud synchronization, failing to consider CloudKit’s limitations during the modeling phase can lead to complex issues later. Below are the essential rules and considerations to ensure smooth integration with CloudKit.
Mandatory Rules for Data Models
1. Unique Constraints
Prohibited:
- Core Data:
Unique constraints - SwiftData:
@Attribute(.unique)
2. Attribute Requirements
- Attributes must not be non-optional without default values.
- Allowed configurations:
- Optional
- With a default value
- Optional + Default value
3. Type Restrictions
- The Core Data
Undefinedtype must not be used.
4. Relationship Requirements
- All relationships must be optional.
- All relationships must include inverse relationships.
- Prohibited configurations:
Denydeletion rule- Ordered relationships (
Ordered)
Key Considerations
Once cloud synchronization is enabled, model changes are limited to lightweight migrations. Developers should plan their data model carefully during the design phase to minimize future modifications.
Model Adjustment Principles
- Do not delete existing entities or attributes.
- Do not rename existing entities or attributes.
Recommendations
Follow the principle of “add only, no deletions or modifications”:
- Only add new entities, attributes, or relationships.