Schema Freezing

Schema freezing is the process of creating immutable snapshots of your SwiftData schemas at specific versions. These snapshots serve as the source of truth for detecting schema drift and testing migrations.

What is Schema Freezing?

When you “freeze” a schema, FreezeRay creates a permanent record of your database structure at that moment in time. This record includes:

SQLite database: A real database file with the exact schema structure
JSON representation: Human-readable schema metadata
SHA256 checksum: A cryptographic hash for drift detection
Export metadata: Timestamp and version information

Think of it like package-lock.json for your database schema - you know exactly what structure shipped with each version.

Why Freeze Schemas?

The Problem: SwiftData’s Hidden Dangers

SwiftData uses internal hashes to match schemas. If you modify a shipped VersionedSchema:

// ❌ DANGER: You modified this after shipping v1
enum AppSchemaV1: VersionedSchema {
    static var models: [any PersistentModel.Type] {
        [User.self, NewModel.self] // Added NewModel after shipping!
    }
}

What happens:

Fresh installs work fine (no migration needed)
Your simulator tests pass (fresh installs)
Production users with existing databases crash on launch

SwiftData’s error messages are cryptic:

Cannot use staged migration with an unknown model version
Persistent store migration failed, missing source managed object model

The Solution: Frozen Fixtures

FreezeRay captures the actual database from each shipped version:

FreezeRay/Fixtures/
├── 1.0.0/
│   ├── App-1_0_0.sqlite       # Real v1.0.0 database
│   ├── schema-1_0_0.json      # Human-readable schema
│   └── export_metadata.txt    # Freeze metadata
└── 2.0.0/
    ├── App-2_0_0.sqlite       # Real v2.0.0 database
    ├── schema-2_0_0.json
    └── export_metadata.txt

These fixtures enable:

Drift detection: Checksum comparison catches accidental changes
Migration testing: Load real v1.0.0 database and test migration to v2.0.0
Audit trail: Git history shows exactly what shipped

How Freezing Works

1. Annotate Your Schema

Add the @FreezeSchema macro with a version string:

import SwiftData
import FreezeRay

@FreezeSchema(version: "1.0.0")
enum AppSchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)

    static var models: [any PersistentModel.Type] {
        [User.self]
    }

    @Model
    final class User {
        var name: String
        var email: String
    }
}

The @FreezeSchema(version:) string is used by FreezeRay for fixture organization. It’s separate from SwiftData’s versionIdentifier.

The macro generates:

__freezeray_freeze_1_0_0() - Exports schema to SQLite
__freezeray_check_1_0_0() - Verifies checksum matches frozen version

2. Run the Freeze Command

freezeray freeze 1.0.0

What happens:

CLI auto-discovers your project (no config file needed)
Parses source files to find @FreezeSchema(version: "1.0.0")
Builds your project
Runs tests in iOS Simulator
Calls the macro-generated freeze function
Extracts SQLite files from simulator container
Generates SHA256 checksums
Creates scaffolded tests for drift detection
Copies fixtures to FreezeRay/Fixtures/1.0.0/

FreezeRay uses SwiftSyntax to automatically discover schemas. No manual configuration needed!

3. Commit to Git

git add FreezeRay/
git commit -m "Freeze schema v1.0.0"

Now your schema is permanently frozen. Any changes trigger drift detection.

Fixture Anatomy

SQLite Database (`App-1_0_0.sqlite`)

A real SQLite database with your schema structure:

-- Actual schema captured from iOS Simulator
CREATE TABLE ZUSER (
    Z_PK INTEGER PRIMARY KEY,
    Z_ENT INTEGER,
    Z_OPT INTEGER,
    ZNAME VARCHAR,
    ZEMAIL VARCHAR
);

CREATE TABLE Z_PRIMARYKEY (
    Z_ENT INTEGER PRIMARY KEY,
    Z_NAME VARCHAR,
    Z_SUPER INTEGER,
    Z_MAX INTEGER
);

-- ... and more SwiftData metadata tables

JSON Schema (`schema-1_0_0.json`)

Human-readable representation:

{
  "version": "1.0.0",
  "models": [
    {
      "name": "User",
      "properties": [
        { "name": "name", "type": "String" },
        { "name": "email", "type": "String" }
      ]
    }
  ],
  "checksum": "0cc298858e409d8b9c7e37a7779af0e2"
}

Export Metadata (`export_metadata.txt`)

Freeze details:

Freeze Date: 2025-11-04T12:34:56Z
Version: 1.0.0
Schema: AppSchemaV1
Models: User
Checksum: 0cc298858e409d8b9c7e37a7779af0e2

When to Freeze

Required: Before Major Schema Changes

Freeze before releasing a new schema version to production:

# Shipping schema V2 with breaking changes
freezeray freeze 2.0.0
git add FreezeRay/
git commit -m "Freeze schema v2.0.0 for App Store release"

Optional: During Development

Freeze interim versions for testing:

# Testing a new schema locally
freezeray freeze 2.0.0-beta.1

Not Required: Every Commit

Don’t freeze on every schema change - only when you want to seal a version as immutable.

Schema Version Numbering

Schema Versions ≠ App Versions

Schema versions are independent from app versions:

// App version 1.0.0 might ship with schema V1
// App version 1.5.0 might STILL use schema V1 (no schema changes)
// App version 2.0.0 might introduce schema V2

Common Patterns

Incremental (Simplest):

@FreezeSchema(version: "1")
enum AppSchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)
}

@FreezeSchema(version: "2")
enum AppSchemaV2: VersionedSchema {
    static var versionIdentifier = Schema.Version(2, 0, 0)
}

Semantic Versioning (More Descriptive):

@FreezeSchema(version: "1.0.0")
enum AppSchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)
}

@FreezeSchema(version: "2.0.0")
enum AppSchemaV2: VersionedSchema {
    static var versionIdentifier = Schema.Version(2, 0, 0)
}

Date-based (Time-ordered):

@FreezeSchema(version: "2024-01-15")
enum AppSchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)
}

Choose a versioning scheme that makes sense for your team. FreezeRay doesn’t enforce any specific format.

Best Practices

Never modify frozen schemas. If you need changes, create a new version (AppSchemaV2).

Always include ALL models in each schema version. Missing models cause “unknown model version” errors.

Migration Plan Structure

Keep your migration plan synchronized with frozen schemas:

enum AppMigrations: SchemaMigrationPlan {
    static var schemas: [any VersionedSchema.Type] {
        [AppSchemaV1.self, AppSchemaV2.self, AppSchemaV3.self]
    }

    static var stages: [MigrationStage] {
        [migrateV1toV2, migrateV2toV3]
    }
}

Type Aliases for Current Schema

Use type aliases to reference the current version throughout your app:

// In your models file
typealias User = DataV3.User
typealias Post = DataV3.Post

// In your app code
@Query var users: [User]  // Uses current version

When you create V4, just update the typealias - no need to change app code.

Next Steps

Drift Detection

Learn how checksums catch accidental schema changes

Migration Testing

Test migrations between frozen versions

First Freeze Guide

Step-by-step tutorial for your first freeze

@FreezeSchema Macro

Macro reference and generated code

Getting Started

Core Concepts

CLI Reference

Macro Reference

Schema Freezing

Schema Freezing

What is Schema Freezing?

Why Freeze Schemas?

The Problem: SwiftData’s Hidden Dangers

The Solution: Frozen Fixtures

How Freezing Works

1. Annotate Your Schema

2. Run the Freeze Command

3. Commit to Git

Fixture Anatomy

SQLite Database (`App-1_0_0.sqlite`)

JSON Schema (`schema-1_0_0.json`)

Export Metadata (`export_metadata.txt`)

When to Freeze

Required: Before Major Schema Changes

Optional: During Development

Not Required: Every Commit

Schema Version Numbering

Schema Versions ≠ App Versions

Common Patterns

Best Practices

Migration Plan Structure

Type Aliases for Current Schema

Next Steps

Drift Detection

Migration Testing

First Freeze Guide

@FreezeSchema Macro

Getting Started

Core Concepts

CLI Reference

Macro Reference

​Schema Freezing

​What is Schema Freezing?

​Why Freeze Schemas?

​The Problem: SwiftData’s Hidden Dangers

​The Solution: Frozen Fixtures

​How Freezing Works

​1. Annotate Your Schema

​2. Run the Freeze Command

​3. Commit to Git

​Fixture Anatomy

​SQLite Database (App-1_0_0.sqlite)

​JSON Schema (schema-1_0_0.json)

​Export Metadata (export_metadata.txt)

​When to Freeze

​Required: Before Major Schema Changes

​Optional: During Development

​Not Required: Every Commit

​Schema Version Numbering

​Schema Versions ≠ App Versions

​Common Patterns

​Best Practices

​Migration Plan Structure

​Type Aliases for Current Schema

​Next Steps

Drift Detection

Migration Testing

First Freeze Guide

@FreezeSchema Macro

Schema Freezing

What is Schema Freezing?

Why Freeze Schemas?

The Problem: SwiftData’s Hidden Dangers

The Solution: Frozen Fixtures

How Freezing Works

1. Annotate Your Schema

2. Run the Freeze Command

3. Commit to Git

Fixture Anatomy

SQLite Database (`App-1_0_0.sqlite`)

JSON Schema (`schema-1_0_0.json`)

Export Metadata (`export_metadata.txt`)

When to Freeze

Required: Before Major Schema Changes

Optional: During Development

Not Required: Every Commit

Schema Version Numbering

Schema Versions ≠ App Versions

Common Patterns

Best Practices

Migration Plan Structure

Type Aliases for Current Schema

Next Steps