Swift Legacy→DQL Migration Guide

Overview

This guide will help you successfully migrate your Ditto Swift application from the legacy query builder APIs to the modern DQL (Ditto Query Language). After reviewing this documentation, you’ll understand how to convert method chaining patterns to DQL syntax and systematically update your data operations.

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I need help migrating a Ditto Swift application from the legacy query builder APIs to modern DQL (Ditto Query Language). This migration involves converting method chaining patterns to SQL-like DQL syntax.

CRITICAL RULES:
1. All query builder method chains (.collection().find()) must be replaced with ditto.store.execute() using DQL
2. Use parameterized queries with :paramName syntax - NEVER string interpolation
3. Counter operations must use PN_INCREMENT BY in APPLY clause - do NOT initialize counter fields
4. Sync subscriptions must use ditto.sync.registerSubscription() instead of .find().subscribe()
5. observeLocal must be replaced with registerObserver

---

CORE MIGRATION AREAS:

1. QUERY SYNTAX MIGRATION

BEFORE (Legacy Query Builder):
```
ditto.store.collection("cars")
    .find("color == $0", args: "red")
    .exec()
```

AFTER (DQL):
```
try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE color = :color",
    arguments: ["color": "red"])
```

2. INSERT OPERATIONS

BEFORE (Legacy Query Builder):
```
ditto.store.collection("cars")
    .upsert(["_id": id, "color": "blue"])
```

AFTER (DQL):
```
try await ditto.store.execute(
    query: "INSERT INTO cars DOCUMENTS (:car)",
    arguments: ["car": carData])
```

3. UPDATE OPERATIONS

BEFORE (Legacy Query Builder):
```
ditto.store.collection("cars")
    .findByID(id)
    .update { doc in
        doc?["color"].set("green")
    }
```

AFTER (DQL):
```
try await ditto.store.execute(
    query: "UPDATE cars SET color = :color WHERE _id = :id",
    arguments: ["color": "green", "id": id])
```

4. DELETE OPERATIONS

BEFORE (Legacy Query Builder):
```
ditto.store.collection("cars").findByID(id).remove()
```

AFTER (DQL):
```
try await ditto.store.execute(
    query: "DELETE FROM cars WHERE _id = :id",
    arguments: ["id": id])
```

5. EVICTION OPERATIONS

BEFORE (Legacy Query Builder):
```
ditto.store.collection("cars").findByID(id).evict()
```

AFTER (DQL):
```
try await ditto.store.execute(
    query: "EVICT FROM cars WHERE _id = :id",
    arguments: ["id": id])
```

6. COUNTER OPERATIONS (PN_COUNTER)

BEFORE (Legacy Query Builder):
```
ditto.store.collection("cars")
    .findByID(id)
    .update { doc in
        doc?["numUpdates"].counter?.increment(by: 1.0)
    }
```

AFTER (DQL with PN_INCREMENT):
```
try await ditto.store.execute(
    query: "UPDATE cars APPLY numUpdates PN_INCREMENT BY :increment WHERE _id = :id",
    arguments: ["increment": 1, "id": id])
```

IMPORTANT: Do NOT initialize counter fields in documents:
```
// WRONG - Creates a register, not a counter
["counter": 0]

// CORRECT - Omit counter field, it's created on first PN_INCREMENT
["_id": id, "color": "blue"]
```

7. DOCUMENT FIELD ACCESS MIGRATION

BEFORE (Legacy Query Builder):
```
let document = ditto.store.collection("cars").findByID(id).exec()
let color = document?.value["color"] as? String
```

AFTER (DQL):
```
let result = try await ditto.store.execute(query: dqlString)
let item = result.items.first
let color = item?.value["color"] as? String
```

8. OBSERVER MIGRATION (observeLocal → registerObserver)

BEFORE (Legacy observeLocal):
```
liveQuery = ditto.store.collection("cars")
    .find("_id.locationId == '\(Constants.locationId)'")
    .observeLocal { docs, event in
        switch event {
        case .update(let changes):
            // Handle changes
        case .initial:
            // Handle initial data
        }
    }
```

AFTER (DQL with registerObserver):
```
observer = ditto.store.registerObserver(
    query: "SELECT * FROM cars WHERE _id.locationId = :locationId",
    arguments: ["locationId": Constants.locationId]
) { result in
    // Extract data within callback — do not pass QueryResultItems outside this scope
    let cars = result.items.compactMap { item -> Car? in
        let car = Car(item.jsonData())
        item.dematerialize()
        return car
    }

    // Update UI on main thread with extracted model objects
    DispatchQueue.main.async {
        self.updateUI(cars)
    }
}

// Don't forget cleanup in deinit
deinit {
    observer?.stop()
}
```

9. SYNC SUBSCRIPTIONS MIGRATION

BEFORE (Legacy Query Builder):
```
let subscription = ditto.store.collection("cars")
    .find("color == $0", args: "red")
    .subscribe()
```

AFTER (DQL):
```
let subscription = try ditto.sync.registerSubscription(
    query: "SELECT * FROM cars WHERE color = :color",
    arguments: ["color": "red"])
```

---

COMMON PITFALLS TO AVOID:

1. DQL Syntax Errors
   - Use :paramName for parameters, not $0 or string interpolation

2. Missing Parameter Binding
   - NEVER use string interpolation in queries
   - Always use parameterized queries with arguments dictionary

3. Counter Type Errors
   - Do NOT initialize counter fields with DittoCounter() or numbers
   - Use PN_INCREMENT BY in APPLY clause
   - Pass negative values for decrements

4. Memory Management with Observers
   - Always store observer reference
   - Call observer?.stop() in deinit

5. Attachment Handling
   - Use ATTACHMENT annotation: "(image ATTACHMENT)"
   - Create attachments with ditto.store.newAttachment()

---

MIGRATION CHECKLIST:

Search for these legacy patterns and replace:
- [ ] .collection( → try await ditto.store.execute(query: "SELECT * FROM
- [ ] .find( → Convert to DQL WHERE clause with arguments
- [ ] .findByID( → Convert to DQL WHERE _id = :id
- [ ] .upsert( → Convert to DQL INSERT INTO
- [ ] .update( → Convert to DQL UPDATE SET
- [ ] .remove( → Convert to DQL DELETE FROM
- [ ] .evict( → Convert to DQL EVICT FROM
- [ ] .counter?.increment( → Convert to PN_INCREMENT BY in APPLY clause
- [ ] DittoCounter() → Remove initialization, use PN_INCREMENT
- [ ] .observeLocal( → Convert to registerObserver
- [ ] .subscribe() → Convert to ditto.sync.registerSubscription()
- [ ] DittoDocument → DittoQueryResultItem
- [ ] DittoSubscription → DittoSyncSubscription

---

Please help me convert all legacy query builder patterns in my codebase to DQL syntax. Focus on:
1. Maintaining the same functionality
2. Using proper parameterized queries with arguments dictionary
3. Handling counter operations correctly with PN_INCREMENT
4. Implementing proper observer cleanup in deinit
5. Converting all sync subscriptions to DQL

Start by identifying all uses of .collection() in my codebase and systematically converting each one to the appropriate DQL pattern.

Syntax Change Reference

Document Query Syntax

Document Query All

try await ditto.store.execute(
    query: "SELECT * FROM cars"

Document Query by ID

try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE _id = '123'"

Document Query with Predicate

try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE color = 'blue'"

Document Query with Arguments

try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE color = :color",
    arguments: ["color": "red"])

Document Insert

try await ditto.store.execute(
    query: "INSERT INTO cars DOCUMENTS (:car)",
    arguments: ["car": carData])

Document Update

try await ditto.store.execute(
    query: "UPDATE cars SET color = :color WHERE _id = :id",
    arguments: ["color": "green", "id": id])

Document Delete

try await ditto.store.execute(
    query: "DELETE FROM cars WHERE _id = :id",
    arguments: ["id": id])

Document Local Eviction

// Evict by ID
try await ditto.store.execute(
    query: "EVICT FROM cars WHERE _id = :id",
    arguments: ["id": id])

// Evict all matching documents
try await ditto.store.execute(
    query: "EVICT FROM cars WHERE color = :color",
    arguments: ["color": "red"])

Query Response Handling

Legacy Query Builder → DQL Response

let result = try await ditto.store.execute(query: dqlString)
let item = result.items.first
let color = item?.value["color"] as? String

Legacy Query Builder → Modern Document Conversion

// Extract data and dematerialize within the same scope
func documentToCar(item: DittoQueryResultItem) -> Car {
    let model = item.value["_id"] as! String
    let color = item.value["color"] as! String

    // Prune to free memory — do not use `item` after this point
    item.dematerialize()

    return Car(id: model, color: color)
}

// Usage — call within the callback or observer scope only
let cars = result.items.compactMap { documentToCar(item: $0) }

Observer Migration

Legacy Query Builder → DQL Store Observer Migration

observer = ditto.store.registerObserver(
    query: "SELECT * FROM cars WHERE _id.locationId = :locationId",
    arguments: ["locationId": Constants.locationId]
) { result in
    // Extract data within callback scope — do not pass QueryResultItems outside this closure
    // Using jsonData() is the most memory-efficient approach in Swift
    let cars = result.items.compactMap { item -> Car? in
        let car = Car(item.jsonData())
        item.dematerialize() // Release native memory
        return car
    }

    // Update UI on main thread with extracted model objects
    DispatchQueue.main.async {
        self.updateUI(cars)
    }
}

Performance Consideration: DQL observers provide more advanced return results including aggregates and projections. This requires more database full scans to ensure consistent results compared to the legacy query builder.Use indexes on query fields to maintain and improve observer performance. Indexes ensure your observers remain functional with optimal query performance.

Best Practice: Create Indexes for Observer Queries

// Create index on frequently queried fields
try await ditto.store.execute("""
    CREATE INDEX idx_cars_locationId
    ON cars (_id.locationId)
""")

// Then register observer - queries will use the index
observer = ditto.store.registerObserver(
    query: "SELECT * FROM cars WHERE _id.locationId = :locationId",
    arguments: ["locationId": Constants.locationId]
) { result in
    // Process results
}

For more information on creating and managing indexes, see the DQL Indexing documentation.

Sync Subscriptions Migration

Legacy Query Builder → DQL Sync Subscriptions Subscribe with Query

let subscription = try ditto.sync.registerSubscription(
    query: "SELECT * FROM cars WHERE color = :color",
    arguments: ["color": "red"])

Subscribe with Parameters

let subscription = try ditto.sync.registerSubscription(
    query: "SELECT * FROM cars WHERE _id.locationId = :locationId",
    arguments: ["locationId": Constants.locationId])

Multiple Subscriptions

var subscriptions: [DittoSyncSubscription] = []
subscriptions.append(
    try ditto.sync.registerSubscription(
        query: "SELECT * FROM cars WHERE color = :color",
        arguments: ["color": "red"])
)
subscriptions.append(
    try ditto.sync.registerSubscription(
        query: "SELECT * FROM cars WHERE year > :year",
        arguments: ["year": 2020])
)

Cancel Subscription

subscription.cancel()

Subscribe to All Documents

let subscription = try ditto.sync.registerSubscription(
    query: "SELECT * FROM cars")

Counter Type Migration

PN_COUNTER is the DQL equivalent of the legacy DittoCounter type. When migrating counter operations from the legacy query builder’s .counter?.increment() method, use PN_INCREMENT BY in the APPLY clause. This maintains full compatibility with existing counter data created by DittoCounter.

Counter Increment

try await ditto.store.execute(
    query: "UPDATE cars APPLY numUpdates PN_INCREMENT BY :increment WHERE _id = :id",
    arguments: ["increment": 1, "id": id])

Counter Decrement

try await ditto.store.execute(
    query: "UPDATE cars APPLY viewCount PN_INCREMENT BY :decrement WHERE _id = :id",
    arguments: ["decrement": -1, "id": id])

Initialize Counter in Document

// Counter fields are automatically created on first PN_INCREMENT use
try await ditto.store.execute(
    query: "INSERT INTO cars DOCUMENTS (:car)",
    arguments: ["car": [
        "_id": id,
        "color": "blue"
        // Do NOT initialize counter fields - they are created on first PN_INCREMENT
    ]])

// Then use PN_INCREMENT with APPLY clause to create and increment the counter
try await ditto.store.execute(
    query: "UPDATE cars APPLY numUpdates PN_INCREMENT BY 1 WHERE _id = :id",
    arguments: ["id": id])

Multiple Counter Operations

try await ditto.store.execute(
    query: """
    UPDATE cars
    APPLY likes PN_INCREMENT BY :likeIncrement,
          dislikes PN_INCREMENT BY :dislikeDecrement,
          views PN_INCREMENT BY :viewIncrement
    WHERE _id = :id
    """,
    arguments: [
        "likeIncrement": 1,
        "dislikeDecrement": -1,
        "viewIncrement": 1,
        "id": id
    ])

Attachment Operations with DQL

Attachment Creation and Storage

// Create attachment using store
let attachment = try ditto.store.newAttachment(
    path: filePath,
    metadata: metadata)

// Store attachment with DQL
try await ditto.store.execute(
    query: "INSERT INTO COLLECTION cars (image ATTACHMENT) DOCUMENTS (:doc)",
    arguments: ["doc": docWithAttachment])

Attachment Fetching

// Fetch attachment with progress callback
let fetchResult = try await ditto.store.fetchAttachment(
    token: attachmentToken,
    onFetchEvent: { event in
        if case let .progress(downloadedBytes, totalBytes) = event {
            updateProgress(downloadedBytes, totalBytes)
        }
    })

Performance Enhancements

Indexes for Improved Query Performance

DQL observers and queries benefit significantly from proper indexing. When migrating from the legacy query builder to DQL, creating indexes on frequently queried fields is essential for maintaining optimal performance. Why Indexes Matter for DQL:

DQL observers support advanced features like aggregates and projections
These advanced features require full database scans to ensure consistent results
Indexes dramatically reduce query execution time by avoiding full scans
Combining indexes with observers provides better performance than legacy query builder

Creating Indexes:

// Create index on single field
try await ditto.store.execute("""
    CREATE INDEX idx_cars_color
    ON cars (color)
""")

// Create compound index on multiple fields
try await ditto.store.execute("""
    CREATE INDEX idx_cars_color_year
    ON cars (color, year)
""")

// Create index on nested field
try await ditto.store.execute("""
    CREATE INDEX idx_cars_location
    ON cars (_id.locationId)
""")

Best Practices:

Create indexes on fields used in WHERE clauses
Create indexes before registering observers for those queries
Use compound indexes for queries with multiple filter conditions
Monitor query performance and add indexes as needed

For comprehensive information on indexing strategies, syntax, and best practices, see the DQL Indexing documentation.

Common Pitfalls to Avoid

1. DQL Syntax Errors

Use :paramName for parameters, not $0 or string interpolation.

// ❌ Wrong: String interpolation
let color = "red"
try await ditto.store.execute(query: "SELECT * FROM cars WHERE color = '\(color)'")

// ✅ Correct: Using :paramName with arguments dictionary
try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE color = :color",
    arguments: ["color": "red"])

2. Missing Parameter Binding

NEVER use string interpolation in queries. Always use parameterized queries with [String: Any] arguments dictionary.

// ❌ Wrong: Direct string interpolation
let locationId = "loc_123"
try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE _id.locationId = '\(locationId)'")

// ✅ Correct: Parameterized query
try await ditto.store.execute(
    query: "SELECT * FROM cars WHERE _id.locationId = :locationId",
    arguments: ["locationId": locationId])

3. Counter Type Errors

Use COUNTER annotation in collection definitions. Do NOT use SET with COUNTER fields. Use APPLY with PN_INCREMENT BY. Pass negative values for decrements.

// ❌ Wrong: Initializing counter with a number (creates REGISTER, not COUNTER)
try await ditto.store.execute(
    query: "INSERT INTO items DOCUMENTS (:doc)",
    arguments: ["doc": ["counter": 0, "_id": id]])

// ❌ Wrong: Using SET on counter field
try await ditto.store.execute(
    query: "UPDATE items SET counter = 5 WHERE _id = :id",
    arguments: ["id": id])

// ✅ Correct: Use PN_INCREMENT BY with APPLY clause (creates counter on first use)
try await ditto.store.execute(
    query: "UPDATE COLLECTION items (counter COUNTER) APPLY counter PN_INCREMENT BY :value WHERE _id = :id",
    arguments: ["value": 1, "id": id])

// ✅ Correct: Decrement by passing negative value
try await ditto.store.execute(
    query: "UPDATE items APPLY counter PN_INCREMENT BY :value WHERE _id = :id",
    arguments: ["value": -1, "id": id])

4. Memory Management with Observers

Extract data immediately within the callback using jsonData() or item.value, then call dematerialize() to free native memory. Always call observer?.stop() in deinit. Use indexes for improved memory and performance.

// ❌ Wrong: Storing QueryResultItems outside callback scope
var items: [DittoQueryResultItem] = []
observer = ditto.store.registerObserver(query: "SELECT * FROM cars") { result in
    self.items = result.items  // Holds native memory
}

// ✅ Correct: Extract data and dematerialize immediately
class ViewController: UIViewController {
    var observer: DittoStoreObserver?
    var cars: [Car] = []

    func setupObserver() {
        observer = ditto.store.registerObserver(
            query: "SELECT * FROM cars",
            arguments: [:]
        ) { result in
            // Extract using jsonData() - most memory-efficient
            let cars = result.items.compactMap { item -> Car? in
                let car = Car(item.jsonData())
                item.dematerialize()  // Free native memory
                return car
            }
            DispatchQueue.main.async { self.cars = cars }
        }
    }

    deinit {
        observer?.stop()  // Always stop observer
    }
}

5. Attachment Handling

Use ATTACHMENT annotation in collection definitions. Create attachments with ditto.store.newAttachment().

// ❌ Wrong: Missing ATTACHMENT annotation
try await ditto.store.execute(
    query: "INSERT INTO cars DOCUMENTS (:doc)",
    arguments: ["doc": docWithAttachment])

// ✅ Correct: Use ATTACHMENT annotation in COLLECTION definition
let attachment = try ditto.store.newAttachment(
    path: filePath,
    metadata: metadata)

try await ditto.store.execute(
    query: "INSERT INTO COLLECTION cars (image ATTACHMENT) DOCUMENTS (:doc)",
    arguments: ["doc": [
        "_id": id,
        "image": attachment
    ]])

Overview & Setup

SDKs

Accessing Data

Syncing Data

Authentication and Authorization

Managing the Mesh

Deploying your App

Swift Legacy→DQL Migration Guide

Overview

AI Agent Prompt

Syntax Change Reference

Document Query Syntax

Query Response Handling

Observer Migration

Sync Subscriptions Migration

Counter Type Migration

Attachment Operations with DQL

Performance Enhancements

Indexes for Improved Query Performance

Common Pitfalls to Avoid

1. DQL Syntax Errors

2. Missing Parameter Binding

3. Counter Type Errors

4. Memory Management with Observers

5. Attachment Handling

Overview & Setup

SDKs

Accessing Data

Syncing Data

Authentication and Authorization

Managing the Mesh

Deploying your App

Documentation Index

​Overview

​AI Agent Prompt

​Syntax Change Reference

​Document Query Syntax

​Query Response Handling

​Observer Migration

​Sync Subscriptions Migration

​Counter Type Migration

​Attachment Operations with DQL

​Performance Enhancements

​Indexes for Improved Query Performance

​Common Pitfalls to Avoid

​1. DQL Syntax Errors

​2. Missing Parameter Binding

​3. Counter Type Errors

​4. Memory Management with Observers

​5. Attachment Handling

Overview

AI Agent Prompt

Syntax Change Reference

Document Query Syntax

Query Response Handling

Observer Migration

Sync Subscriptions Migration

Counter Type Migration

Attachment Operations with DQL

Performance Enhancements

Indexes for Improved Query Performance

Common Pitfalls to Avoid

1. DQL Syntax Errors

2. Missing Parameter Binding

3. Counter Type Errors

4. Memory Management with Observers

5. Attachment Handling