Allow disabling conflict target (#2925)

drift/lib/src/runtime/query_builder/statements/insert.dart

@@ -276,8 +276,15 @@ class InsertStatement<T extends Table, D> {
       if (ctx.dialect == SqlDialect.mariadb) {
         ctx.buffer.write(' ON DUPLICATE');
       } else {
-        ctx.buffer.write(' ON CONFLICT(');
+        ctx.buffer.write(' ON CONFLICT');
 
+        if (target != null && target.isEmpty) {
+          // An empty list indicates that no explicit target should be generated
+          // by drift, the default rules by the database will apply instead.
+          return;
+        }
+
+        ctx.buffer.write('(');
         final conflictTarget = target ?? table.$primaryKey.toList();
 
         if (conflictTarget.isEmpty) {
@@ -473,6 +480,8 @@ class DoUpdate<T extends Table, D> extends UpsertClause<T, D> {
   /// specifies the uniqueness constraint that will trigger the upsert.
   ///
   /// By default, the primary key of the table will be used.
+  /// This can be set to an empty list, in which case no explicit conflict
+  /// target will be generated by drift.
   final List<Column>? target;
 
   /// Creates a `DO UPDATE` clause.

drift/test/database/statements/insert_test.dart

@@ -263,6 +263,22 @@ void main() {
     ));
   });
 
+  test('can ignore conflict target', () async {
+    await db.into(db.todosTable).insert(
+          TodosTableCompanion.insert(content: 'my content'),
+          onConflict: DoUpdate((old) {
+            return TodosTableCompanion.custom(
+                content: const Variable('important: ') + old.content);
+          }, target: []),
+        );
+
+    verify(executor.runInsert(
+      'INSERT INTO "todos" ("content") VALUES (?) '
+      'ON CONFLICT DO UPDATE SET "content" = ? || "content"',
+      argThat(equals(['my content', 'important: '])),
+    ));
+  });
+
   test(
     'can use multiple upsert targets',
     () async {