feat: Support specifying the conflicting bias

lib/src/database/triggers.dart

@@ -1,5 +1,6 @@
 import 'package:drift/drift.dart';
 
+import '../migrator.dart';
 import '../utils/sql_builder.dart';
 
 /// The name of the HLC function used in the database.
@@ -15,7 +16,13 @@ enum Operation { insert, update, delete }
 /// Provides the CRDT control trigger statements for the database.
 abstract class OfflineSyncTriggers extends CrdtDataSqlBuilder {
   /// Creates a new instance of [OfflineSyncTriggers].
-  OfflineSyncTriggers(super.crdtDb);
+  OfflineSyncTriggers(
+    super.crdtDb, [
+    this.conflictingBias = ConflictingBias.deleteWins,
+  ]);
+
+  /// The bias to use when conflicting operations are detected.
+  final ConflictingBias conflictingBias;
 
   /// Wraps the provided SQL statement as a CREATE TRIGGER statement.
   ///
@@ -52,14 +59,23 @@ abstract class OfflineSyncTriggers extends CrdtDataSqlBuilder {
       tableAlias: operation == Operation.insert ? 'NEW' : 'OLD',
     );
 
-    final columnNames = operation == Operation.delete
+    final columnNames = [
-        ? [isDeletedColumnName]
+      if (operation != Operation.delete) ...table.nonGenerated,
-        : table.nonGenerated;
+      if (operation != Operation.update ||
+          conflictingBias == ConflictingBias.updateWins)
+        isDeletedColumnName,
+    ];
 
     final columnInserts = columnNames.map((c) {
+      final columnValue = c == isDeletedColumnName
+          ? operation == Operation.delete
+                ? 'TRUE'
+                : 'FALSE'
+          : 'NEW."$c"';
+
       return '    SELECT\n${[
         '      \'$c\' AS "column_name"',
-        '      ${c == isDeletedColumnName ? 'TRUE' : 'NEW."$c"'} AS "raw_value"',
+        '      $columnValue AS "raw_value"',
         if (operation == Operation.update) '      NEW."$c" IS NOT OLD."$c" AS "value_changed"',
       ].join(',\n')}';
     });
@@ -111,7 +127,7 @@ ${columnInserts.join('\n    UNION ALL\n')}
 /// Provides the CRDT control trigger statements for the SQLite3 database.
 class Sqlite3OfflineSyncTriggers extends OfflineSyncTriggers {
   /// Creates a new instance of [Sqlite3OfflineSyncTriggers].
-  Sqlite3OfflineSyncTriggers(super.crdtDb);
+  Sqlite3OfflineSyncTriggers(super.crdtDb, [super.conflictingBias]);
 
   @override
   String wrapAsCreateTriggerStatement({

lib/src/migrator.dart

@@ -4,6 +4,15 @@ import 'crdt/crdt.dart';
 import 'database/database.dart';
 import 'database/triggers.dart';
 
+/// The bias to use when conflicting operations are detected.
+enum ConflictingBias {
+  /// Newer updates are overridden by deletes.
+  deleteWins,
+
+  /// Deletes are undone by newer conflicting updates.
+  updateWins,
+}
+
 /// Runs migrations declared by a [MigrationStrategy].
 class OfflineSyncMigrator extends Migrator {
   /// Used internally by drift when opening the database.
@@ -11,6 +20,7 @@ class OfflineSyncMigrator extends Migrator {
     super.database, {
     required this.userId,
     required this.nodeId,
+    this.conflictingBias = ConflictingBias.deleteWins,
 
     /// Restrict tables to synchronize. If not provided, all tables will be considered.
     List<TableInfo>? synchronizedTables,
@@ -38,6 +48,11 @@ class OfflineSyncMigrator extends Migrator {
   /// Tables to be synchronized with CRDT.
   final List<TableInfo> synchronizedTables;
 
+  /// The bias to use when conflicting operations are detected.
+  ///
+  /// The default is [ConflictingBias.deleteWins], which is the most common case.
+  final ConflictingBias conflictingBias;
+
   /// The database instance to apply the CRDT schema to.
   late final crdtDb = CrdtDatabase(
     database.executor,
@@ -103,7 +118,7 @@ class OfflineSyncMigrator extends Migrator {
 
   Future<void> _createTriggers(TableInfo table) async {
     final triggerCreator = switch (database.executor.dialect) {
-      SqlDialect.sqlite => Sqlite3OfflineSyncTriggers(crdtDb),
+      SqlDialect.sqlite => Sqlite3OfflineSyncTriggers(crdtDb, conflictingBias),
       // TODO: Implement the triggers for the Postgres dialect.
       _ => throw UnsupportedError('Unsupported dialect: ${database.executor.dialect}'),
     };

test/crdt/crdt_data_convert_test.dart

@@ -72,7 +72,11 @@ void main() {
       // Perform an insert to the database to get the exact changeset that would
       // be generated by the CRDT triggers.
       await database.tableWithEveryColumnType.insertOne(createdData);
-      expectedChangeset = await crdtDb.crdtDataTable.all().get();
+      expectedChangeset = await crdtDb.managers.crdtDataTable
+          .filter(
+            (f) => f.columnName.equals(crdtDb.sqlBuilder.isDeletedColumnName).not(),
+          )
+          .get();
       testHlc = expectedChangeset.first.hlcTimestamp;
       await crdtDb.crdtDataTable.deleteAll();
     });

test/crdt/crdt_get_changeset_test.dart

@@ -59,7 +59,10 @@ void main() {
           () async {
             final changeset = await crdt.getChangeset();
             final changesetColumns = changeset.map((e) => e.columnName).toSet();
-            final expectedColumns = database.todosTable.$columns.map((c) => c.$name);
+            final expectedColumns = [
+              ...database.todosTable.$columns.map((c) => c.$name),
+              crdtDb.sqlBuilder.isDeletedColumnName,
+            ];
 
             expect(changeset.length, expectedColumns.length);
             expect(changesetColumns, equals(expectedColumns));

test/triggers/trigger_execution_test.dart

@@ -33,17 +33,22 @@ void main() {
       });
 
       group('then the trigger is executed and the CRDT data table', () {
+        late String isDeletedColumnName;
         late List<CrdtDataEntry> allCrdtDataEntries;
 
         setUp(() async {
           final migrator = database.createMigrator();
           allCrdtDataEntries = await migrator.crdtDb.managers.crdtDataTable.get();
+          isDeletedColumnName = migrator.crdtDb.sqlBuilder.isDeletedColumnName;
         });
 
         test('has one entry for each column of the target table.', () async {
           expect(
             allCrdtDataEntries.map((e) => e.columnName).toSet(),
-            equals(database.todosTable.$columns.map((c) => c.$name).toSet()),
+            equals(
+              database.todosTable.$columns.map((c) => c.$name).toSet()
+                ..add(isDeletedColumnName),
+            ),
           );
         });
 
@@ -81,11 +86,11 @@ void main() {
       createdRowId = await database.managers.todosTable.create(
         (t) => t(content: initialContent),
       );
+      rowId = createdRowId.toString();
 
       final migrator = database.createMigrator();
       final allCrdtDataEntries = await migrator.crdtDb.managers.crdtDataTable.get();
       createdHlc = allCrdtDataEntries.first.hlcTimestamp;
-      rowId = createdRowId.toString();
     });
 
     group('when updating the existing row with a different value', () {
@@ -206,24 +211,23 @@ void main() {
         'then the trigger is executed and the deleted row in the CRDT data table',
         () {
           late List<CrdtDataEntry> allCrdtDataEntries;
-          late String rowId;
+          late String isDeletedColumnName;
 
           setUp(() async {
             final migrator = database.createMigrator();
             allCrdtDataEntries = await migrator.crdtDb.managers.crdtDataTable.get();
-
+            isDeletedColumnName = migrator.crdtDb.sqlBuilder.isDeletedColumnName;
-            rowId = createdRowId.toString();
           });
 
           test('has the deleted flag column set to true.', () async {
-            final crdtData = allCrdtDataEntries.getField(rowId, '__crdt_is_deleted');
+            final crdtData = allCrdtDataEntries.getField(rowId, isDeletedColumnName);
             expect(crdtData.unwrappedValue, 1);
           });
 
           test('has the deleted HLC timestamp greater than the created.', () async {
             final deletedEntry = allCrdtDataEntries.getField(
               rowId,
-              '__crdt_is_deleted',
+              isDeletedColumnName,
             );
             expect(deletedEntry.hlcTimestamp, greaterThan(createdHlc));
           });
@@ -231,6 +235,93 @@ void main() {
       );
     });
   });
+
+  group('Given a database with a previously deleted row from a synchronized table', () {
+    const initialContent = 'test';
+    late String isDeletedColumnName;
+    late int createdRowId;
+    late Hlc deletedHlc;
+    late String rowId;
+
+    setUp(() async {
+      createdRowId = await database.managers.todosTable.create(
+        (t) => t(content: initialContent),
+      );
+      rowId = createdRowId.toString();
+
+      final migrator = database.createMigrator();
+
+      final deleted = await database.managers.todosTable
+          .filter((t) => t.id.equals(RowId(createdRowId)))
+          .delete();
+      expect(deleted, equals(1));
+
+      final allCrdtDataEntries = await migrator.crdtDb.managers.crdtDataTable.get();
+      isDeletedColumnName = migrator.crdtDb.sqlBuilder.isDeletedColumnName;
+      deletedHlc = allCrdtDataEntries.getField(rowId, isDeletedColumnName).hlcTimestamp;
+    });
+
+    group('when re-inserting the previously deleted row', () {
+      setUp(() async {
+        await database.managers.todosTable.create(
+          (t) => t(
+            id: Value(RowId(createdRowId)),
+            content: initialContent,
+          ),
+        );
+      });
+
+      test(
+        'then the row has been re-inserted in the target table with the same data.',
+        () async {
+          final row = await database.managers.todosTable.getSingle();
+
+          expect(row, isNotNull);
+          expect(row.id, equals(createdRowId));
+          expect(row.content, initialContent);
+        },
+      );
+
+      group(
+        'then the trigger is executed and the re-inserted row in the CRDT data table',
+        () {
+          late List<CrdtDataEntry> allCrdtDataEntries;
+
+          setUp(() async {
+            final migrator = database.createMigrator();
+            allCrdtDataEntries = await migrator.crdtDb.managers.crdtDataTable.get();
+            isDeletedColumnName = migrator.crdtDb.sqlBuilder.isDeletedColumnName;
+          });
+
+          test('has the deleted flag column set to false.', () async {
+            final crdtData = allCrdtDataEntries.getField(rowId, isDeletedColumnName);
+            expect(crdtData.unwrappedValue, 0);
+          });
+
+          test(
+            'has the deleted flag HLC timestamp greater than the deleted.',
+            () async {
+              final deletedEntry = allCrdtDataEntries.getField(
+                rowId,
+                isDeletedColumnName,
+              );
+              expect(deletedEntry.hlcTimestamp, greaterThan(deletedHlc));
+            },
+          );
+
+          test(
+            'has the HLC timestamp updated for newly inserted columns.',
+            () async {
+              expect(
+                allCrdtDataEntries.map((e) => e.hlcTimestamp),
+                everyElement(greaterThan(deletedHlc)),
+              );
+            },
+          );
+        },
+      );
+    });
+  });
 }
 
 extension on Iterable<CrdtDataEntry> {

test/triggers/trigger_generation_test.dart

@@ -74,6 +74,10 @@ BEGIN
     SELECT
       'status' AS "column_name",
       NEW."status" AS "raw_value"
+    UNION ALL
+    SELECT
+      '__crdt_is_deleted' AS "column_name",
+      FALSE AS "raw_value"
   )
   WHERE TRUE
   ON CONFLICT ("user_id", "table_name", "column_name", "row_id")
@@ -312,4 +316,60 @@ END;''',
       });
     });
   });
+
+  // This test assures that the above tests are for [ConflictingBias.deleteWins]
+  // and avoids the need for duplicating all tests.
+  test('Given a SQLite3 migrator with no specified conflicting bias '
+      'then it generates the triggers with the deleteWins conflicting bias.', () {
+    final migrator = database.createMigrator();
+    expect(migrator.conflictingBias, ConflictingBias.deleteWins);
+  });
+
+  group('Given a SQLite3 database with a table and updateWins conflicting bias', () {
+    late OfflineSyncMigrator migrator;
+    late String isDeletedColumnName;
+
+    setUp(() {
+      migrator = OfflineSyncMigrator(
+        database,
+        userId: testUserId,
+        nodeId: testNodeId,
+        conflictingBias: ConflictingBias.updateWins,
+        synchronizedTables: [
+          database.todosTable,
+        ],
+      );
+
+      isDeletedColumnName = migrator.crdtDb.sqlBuilder.isDeletedColumnName;
+    });
+
+    group('when generating trigger statements', () {
+      late List<String> triggers;
+
+      setUp(() {
+        final generator = Sqlite3OfflineSyncTriggers(
+          migrator.crdtDb,
+          ConflictingBias.updateWins,
+        );
+        triggers = generator.generateCreateTriggerStatements(database.todosTable);
+      });
+
+      test('then the update trigger includes the deleted flag column.', () {
+        final updateStatement = triggers.firstWhere(
+          (trigger) => trigger.contains('AFTER UPDATE ON "todos"'),
+        );
+
+        expect(
+          updateStatement,
+          contains('''
+    UNION ALL
+    SELECT
+      '$isDeletedColumnName' AS "column_name",
+      FALSE AS "raw_value",
+      NEW."__crdt_is_deleted" IS NOT OLD."__crdt_is_deleted" AS "value_changed"
+  '''),
+        );
+      });
+    });
+  });
 }