> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ditto.live/llms.txt
> Use this file to discover all available pages before exploring further.

# Flutter Legacy→DQL Migration Guide

> Guide for migrating Flutter apps from legacy query builder APIs to DQL (Ditto Query Language) syntax.

## Overview

This guide will help you successfully migrate your Ditto Flutter 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.

## AI Agent Prompt

Use this prompt when working with an AI coding assistant to migrate your Ditto Flutter app from legacy query builder to DQL.

<Accordion title="Copy AI Migration Prompt (Click to Expand)">
  ````text theme={null}
  I need help migrating a Ditto Flutter 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):
  ```dart
  ditto.store.collection('cars')
    .find('color == \$args.color', arguments: {'color': 'red'})
    .exec();
  ```

  AFTER (DQL):
  ```dart
  await ditto.store.execute(
    'SELECT * FROM cars WHERE color = :color',
    arguments: {'color': 'red'}
  );
  ```

  2. INSERT OPERATIONS

  BEFORE (Legacy Query Builder):
  ```dart
  ditto.store.collection('cars')
    .upsert({'_id': id, 'color': 'blue'});
  ```

  AFTER (DQL):
  ```dart
  await ditto.store.execute(
    'INSERT INTO cars DOCUMENTS (:car)',
    arguments: {'car': carData}
  );
  ```

  3. UPDATE OPERATIONS

  BEFORE (Legacy Query Builder):
  ```dart
  ditto.store.collection('cars')
    .findById(id)
    .update((doc) {
      doc['color'] = 'green';
    });
  ```

  AFTER (DQL):
  ```dart
  await ditto.store.execute(
    'UPDATE cars SET color = :color WHERE _id = :id',
    arguments: {'color': 'green', 'id': id}
  );
  ```

  4. DELETE OPERATIONS

  BEFORE (Legacy Query Builder):
  ```dart
  ditto.store.collection('cars').findById(id).remove();
  ```

  AFTER (DQL):
  ```dart
  await ditto.store.execute(
    'DELETE FROM cars WHERE _id = :id',
    arguments: {'id': id}
  );
  ```

  5. EVICTION OPERATIONS

  BEFORE (Legacy Query Builder):
  ```dart
  ditto.store.collection('cars').findById(id).evict();
  ```

  AFTER (DQL):
  ```dart
  await ditto.store.execute(
    'EVICT FROM cars WHERE _id = :id',
    arguments: {'id': id}
  );
  ```

  6. COUNTER OPERATIONS (PN_COUNTER)

  BEFORE (Legacy Query Builder):
  ```dart
  ditto.store.collection('cars')
    .findById(id)
    .update((doc) {
      doc['numUpdates'].counter?.increment(1);
    });
  ```

  AFTER (DQL with PN_INCREMENT):
  ```dart
  await ditto.store.execute(
    'UPDATE cars APPLY numUpdates PN_INCREMENT BY :increment WHERE _id = :id',
    arguments: {'increment': 1, 'id': id}
  );
  ```

  IMPORTANT: Do NOT initialize counter fields in documents:
  ```dart
  // 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):
  ```dart
  final document = ditto.store.collection('cars').findById(id).exec();
  final color = document?.value['color'];
  ```

  AFTER (DQL):
  ```dart
  final result = await ditto.store.execute(dqlString);
  final item = result.items.first;
  final color = item.value['color'];
  ```

  8. OBSERVER MIGRATION (observeLocal → registerObserver)

  BEFORE (Legacy observeLocal):
  ```dart
  final liveQuery = ditto.store.collection('cars')
    .find('_id.locationId == "${Constants.locationId}"')
    .observeLocal((docs, event) {
      if (event is DittoLiveQueryUpdate) {
        // Handle changes
      } else if (event is DittoLiveQueryInitial) {
        // Handle initial data
      }
    });
  ```

  AFTER (DQL with registerObserver):
  ```dart
  final subscription = ditto.store.registerObserver(
    'SELECT * FROM cars WHERE _id.locationId = :locationId',
    arguments: {'locationId': Constants.locationId},
    onChanged: (result) {
      // Process result items
      final items = result.items;

      // Update UI
      setState(() {
        _cars = items;
      });
    },
  );
  ```

  9. SYNC SUBSCRIPTIONS MIGRATION

  BEFORE (Legacy Query Builder):
  ```dart
  final subscription = ditto.store.collection('cars')
    .find('color == \$args.color', arguments: {'color': 'red'})
    .subscribe();
  ```

  AFTER (DQL):
  ```dart
  final subscription = ditto.sync.registerSubscription(
    'SELECT * FROM cars WHERE color = :color',
    arguments: {'color': 'red'}
  );
  ```

  ---

  COMMON PITFALLS TO AVOID:

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

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

  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
     - Store observer reference
     - Call subscription.cancel() in dispose()

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

  ---

  MIGRATION CHECKLIST:

  Search for these legacy patterns and replace:
  - [ ] .collection( → await ditto.store.execute('SELECT * FROM
  - [ ] .find( → Convert to DQL WHERE clause with arguments
  - [ ] .findById( → Convert to DQL WHERE _id = :id
  - [ ] .upsert( → Convert to DQL INSERT INTO COLLECTION
  - [ ] .update( → Convert to DQL UPDATE COLLECTION 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()

  ---

  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 map
  3. Handling counter operations correctly with PN_INCREMENT
  4. Implementing proper observer cleanup in dispose()
  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.
  ````
</Accordion>

***

## Syntax Change Reference

### Document Query Syntax

**Document Query All**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'SELECT * FROM cars'
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars').exec();
  ```
</CodeGroup>

**Document Query by ID**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    "SELECT * FROM cars WHERE _id = '123'"
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .findById('123')
    .exec();
  ```
</CodeGroup>

**Document Query with Predicate**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    "SELECT * FROM cars WHERE color = 'blue'"
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .find("color == 'blue'")
    .exec();
  ```
</CodeGroup>

**Document Query with Arguments**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'SELECT * FROM cars WHERE color = :color',
    arguments: {'color': 'red'}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .find('color == \$args.color', arguments: {'color': 'red'})
    .exec();
  ```
</CodeGroup>

**Document Insert**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'INSERT INTO cars DOCUMENTS (:car)',
    arguments: {'car': carData}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .upsert({'_id': id, 'color': 'blue'});
  ```
</CodeGroup>

**Document Update**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'UPDATE cars SET color = :color WHERE _id = :id',
    arguments: {'color': 'green', 'id': id}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  // Upsert document
  ditto.store.collection('cars')
    .upsert({'_id': id, 'color': 'green'});

  // Update with closure
  ditto.store.collection('cars')
    .findById(id)
    .update((doc) {
      doc['color'] = 'green';
    });
  ```
</CodeGroup>

**Document Delete**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'DELETE FROM cars WHERE _id = :id',
    arguments: {'id': id}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars').findById(id).remove();
  ```
</CodeGroup>

**Document Local Eviction**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // Evict by ID
  await ditto.store.execute(
    'EVICT FROM cars WHERE _id = :id',
    arguments: {'id': id}
  );

  // Evict all matching documents
  await ditto.store.execute(
    'EVICT FROM cars WHERE color = :color',
    arguments: {'color': 'red'}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  // Evict by ID
  ditto.store.collection('cars').findById(id).evict();

  // Evict all matching documents
  ditto.store.collection('cars').findAll().evict();
  ```
</CodeGroup>

### Query Response Handling

**Legacy Query Builder → DQL Response**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  final result = await ditto.store.execute(dqlString);
  final item = result.items.first;
  final color = item.value['color'];
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final document = ditto.store.collection('cars').findById(id).exec();
  final color = document?.value['color'];
  ```
</CodeGroup>

**Legacy Query Builder → Modern Document Conversion**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  Car documentToCar(DittoQueryResultItem item) {
    return Car(
      id: item.value['_id'] as String,
      color: item.value['color'] as String,
    );
  }
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  Car documentToCar(DittoDocument doc) {
    return Car(
      id: doc.value['_id'] as String,
      color: doc.value['color'] as String,
    );
  }
  ```
</CodeGroup>

### Observer Migration

**Legacy Query Builder → DQL Store Observer Migration**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  final subscription = ditto.store.registerObserver(
    'SELECT * FROM cars WHERE _id.locationId = :locationId',
    arguments: {'locationId': Constants.locationId},
    onChanged: (result) {
      // Process result items
      final items = result.items;

      // Update UI
      setState(() {
        _cars = items;
      });
    },
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final liveQuery = ditto.store.collection('cars')
    .find('_id.locationId == "${Constants.locationId}"')
    .observeLocal((docs, event) {
      print('Live query handler called. Event: $event Docs count: ${docs.length}');
      if (event is DittoLiveQueryUpdate) {
        // Handle deletions, insertions, updates
      } else if (event is DittoLiveQueryInitial) {
        // Handle initial data
      }
    });
  ```
</CodeGroup>

<Warning>
  **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.
</Warning>

**Best Practice: Create Indexes for Observer Queries**

```dart theme={null}
// Create index on frequently queried fields
await ditto.store.execute('''
  CREATE INDEX idx_cars_locationId
  ON cars (_id.locationId)
''');

// Then register observer - queries will use the index
final subscription = ditto.store.registerObserver(
  'SELECT * FROM cars WHERE _id.locationId = :locationId',
  arguments: {'locationId': Constants.locationId},
  onChanged: (result) {
    // Process results
  },
);
```

For more information on creating and managing indexes, see the [DQL Indexing documentation](/dql/indexing).

### Sync Subscriptions Migration

**Legacy Query Builder → DQL Sync Subscriptions**

**Subscribe with Query**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  final subscription = ditto.sync.registerSubscription(
    'SELECT * FROM cars WHERE color = :color',
    arguments: {'color': 'red'}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final subscription = ditto.store.collection('cars')
    .find('color == \$args.color', arguments: {'color': 'red'})
    .subscribe();
  ```
</CodeGroup>

**Subscribe with Parameters**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  final subscription = ditto.sync.registerSubscription(
    'SELECT * FROM cars WHERE _id.locationId = :locationId',
    arguments: {'locationId': Constants.locationId}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final subscription = ditto.store.collection('cars')
    .find('_id.locationId == "${Constants.locationId}"')
    .subscribe();
  ```
</CodeGroup>

**Multiple Subscriptions**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  final subscriptions = <DittoSyncSubscription>[];
  subscriptions.add(
    ditto.sync.registerSubscription(
      'SELECT * FROM cars WHERE color = :color',
      arguments: {'color': 'red'}
    )
  );
  subscriptions.add(
    ditto.sync.registerSubscription(
      'SELECT * FROM cars WHERE year > :year',
      arguments: {'year': 2020}
    )
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final subscriptions = <DittoSubscription>[];
  subscriptions.add(
    ditto.store.collection('cars')
      .find("color == 'red'")
      .subscribe()
  );
  subscriptions.add(
    ditto.store.collection('cars')
      .find('year > 2020')
      .subscribe()
  );
  ```
</CodeGroup>

**Cancel Subscription**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  subscription.cancel();
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  subscription.cancel();
  ```
</CodeGroup>

**Subscribe to All Documents**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  final subscription = ditto.sync.registerSubscription(
    'SELECT * FROM cars'
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final subscription = ditto.store.collection('cars')
    .findAll()
    .subscribe();
  ```
</CodeGroup>

### Counter Type Migration

<Note>
  **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`.
</Note>

**Counter Increment**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'UPDATE cars APPLY numUpdates PN_INCREMENT BY :increment WHERE _id = :id',
    arguments: {'increment': 1, 'id': id}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .findById(id)
    .update((doc) {
      doc['numUpdates'].counter?.increment(1);
    });
  ```
</CodeGroup>

**Counter Decrement**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    'UPDATE cars APPLY viewCount PN_INCREMENT BY :decrement WHERE _id = :id',
    arguments: {'decrement': -1, 'id': id}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .findById(id)
    .update((doc) {
      doc['viewCount'].counter?.increment(-1);
    });
  ```
</CodeGroup>

**Initialize Counter in Document**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // Counter fields are automatically created on first PN_INCREMENT use
  await ditto.store.execute(
    '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
  await ditto.store.execute(
    'UPDATE cars APPLY numUpdates PN_INCREMENT BY 1 WHERE _id = :id',
    arguments: {'id': id}
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  final carData = {
    '_id': id,
    'color': 'blue',
    'numUpdates': DittoCounter()
  };
  ditto.store.collection('cars').upsert(carData);
  ```
</CodeGroup>

**Multiple Counter Operations**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  await ditto.store.execute(
    '''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
    }
  );
  ```

  ```dart FLUTTER LEGACY (v4) theme={null}
  ditto.store.collection('cars')
    .findById(id)
    .update((doc) {
      doc['likes'].counter?.increment(1);
      doc['dislikes'].counter?.increment(-1);
      doc['views'].counter?.increment(1);
    });
  ```
</CodeGroup>

### Attachment Operations with DQL

**Attachment Creation and Storage**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // Create attachment using store
  final attachment = await ditto.store.newAttachment(
    filePath,
    metadata: metadata
  );

  // Store attachment with DQL
  await ditto.store.execute(
    'INSERT INTO COLLECTION cars (image ATTACHMENT) DOCUMENTS (:doc)',
    arguments: {'doc': docWithAttachment}
  );
  ```
</CodeGroup>

**Attachment Fetching**

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // Fetch attachment with progress callback
  final fetchResult = await ditto.store.fetchAttachment(
    attachmentToken,
    onProgress: (downloadedBytes, totalBytes) {
      updateProgress(downloadedBytes, totalBytes);
    }
  );
  ```
</CodeGroup>

***

## 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:**

```dart theme={null}
// Create index on single field
await ditto.store.execute('''
  CREATE INDEX idx_cars_color
  ON cars (color)
''');

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

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

**Best Practices:**

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

For comprehensive information on indexing strategies, syntax, and best practices, see the [DQL Indexing documentation](/dql/indexing).

***

## Common Pitfalls to Avoid

### 1. DQL Syntax Errors

Use `:paramName` for parameters, not `\$args.paramName` or string interpolation.

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // ❌ Wrong: String interpolation
  final color = 'red';
  await ditto.store.execute('SELECT * FROM cars WHERE color = \'$color\'');

  // ✅ Correct: Using :paramName with Map<String, dynamic>
  await ditto.store.execute(
    'SELECT * FROM cars WHERE color = :color',
    arguments: {'color': 'red'}
  );
  ```
</CodeGroup>

### 2. Missing Parameter Binding

**NEVER** use string interpolation in queries. Always use parameterized queries with `Map<String, dynamic>`.

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // ❌ Wrong: Direct string interpolation
  final locationId = 'loc_123';
  await ditto.store.execute(
    'SELECT * FROM cars WHERE _id.locationId = \'$locationId\''
  );

  // ✅ Correct: Parameterized query
  await ditto.store.execute(
    'SELECT * FROM cars WHERE _id.locationId = :locationId',
    arguments: {'locationId': locationId}
  );
  ```
</CodeGroup>

### 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.

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // ❌ Wrong: Initializing counter with a number (creates REGISTER, not COUNTER)
  await ditto.store.execute(
    'INSERT INTO items DOCUMENTS (:doc)',
    arguments: {'doc': {'counter': 0, '_id': id}}
  );

  // ❌ Wrong: Using SET on counter field
  await ditto.store.execute(
    'UPDATE items SET counter = 5 WHERE _id = :id',
    arguments: {'id': id}
  );

  // ✅ Correct: Use PN_INCREMENT BY with APPLY clause (creates counter on first use)
  await ditto.store.execute(
    '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
  await ditto.store.execute(
    'UPDATE items APPLY counter PN_INCREMENT BY :value WHERE _id = :id',
    arguments: {'value': -1, 'id': id}
  );
  ```
</CodeGroup>

### 4. Memory Management with Observers

Call `item.close()` after extracting data from QueryResultItems. Always cancel observers when disposing. Use indexes for improved memory and performance.

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // ❌ Wrong: Storing QueryResultItems outside callback scope
  class CarRepository {
    List<DittoQueryResultItem> _items = [];

    void setupObserver() {
      _observer = ditto.store.registerObserver(
        'SELECT * FROM cars',
        (result) {
          _items = result.items;  // Holds native memory
        }
      );
    }
  }

  // ✅ Correct: Extract data and close items immediately
  class CarRepository extends ChangeNotifier {
    DittoStoreObserver? _observer;
    List<String> _carIds = [];

    void startObserving() {
      _observer = ditto.store.registerObserver(
        'SELECT * FROM cars',
        (result) {
          final ids = <String>[];
          for (final item in result.items) {
            final id = item.value['_id'] as String;
            item.close();  // Free native memory
            ids.add(id);
          }
          _carIds = ids;
          notifyListeners();
        }
      );
    }

    @override
    void dispose() {
      _observer?.cancel();  // Always cancel observer
      super.dispose();
    }
  }
  ```
</CodeGroup>

### 5. Attachment Handling

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

<CodeGroup>
  ```dart FLUTTER DQL (v5) theme={null}
  // ❌ Wrong: Missing ATTACHMENT annotation
  await ditto.store.execute(
    'INSERT INTO cars DOCUMENTS (:doc)',
    arguments: {'doc': docWithAttachment}
  );

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

  await ditto.store.execute(
    'INSERT INTO COLLECTION cars (image ATTACHMENT) DOCUMENTS (:doc)',
    arguments: {
      'doc': {
        '_id': id,
        'image': attachment
      }
    }
  );
  ```
</CodeGroup>