diff --git a/lib/src/mock_supabase_http_client.dart b/lib/src/mock_supabase_http_client.dart
index ebeab1c..b354ec5 100644
--- a/lib/src/mock_supabase_http_client.dart
+++ b/lib/src/mock_supabase_http_client.dart
@@ -434,13 +434,27 @@ class MockSupabaseHttpClient extends BaseClient {
         ? List<Map<String, dynamic>>.from(data)
         : [Map<String, dynamic>.from(data)];
 
+    // Get the onConflict columns
+    final onConflictColumns =
+        (request.url.queryParameters['on_conflict'] ?? 'id')
+            .split(',')
+            .map((e) => e.trim())
+            .toList();
+
     // Upsert each item
     final results = items.map((item) {
-      final id = item['id'];
-      if (id != null) {
-        final index =
-            _database[tableKey]!.indexWhere((dbItem) => dbItem['id'] == id);
+      // Check if all onConflictColumns are set in item
+      final shouldUpdate =
+          onConflictColumns.every((column) => item[column] != null);
+
+      if (shouldUpdate) {
+        // Find the index for an item that matches all onConflictColumns
+        final index = _database[tableKey]!.indexWhere((dbItem) =>
+            onConflictColumns
+                .every((column) => dbItem[column] == item[column]));
+
         if (index != -1) {
+          // Update the item in the database
           _database[tableKey]![index] = {
             ..._database[tableKey]![index],
             ...item
diff --git a/test/mock_supabase_http_client_test.dart b/test/mock_supabase_http_client_test.dart
index 63a158d..5917eaa 100644
--- a/test/mock_supabase_http_client_test.dart
+++ b/test/mock_supabase_http_client_test.dart
@@ -59,6 +59,49 @@ void main() {
       expect(postsUfterUpdate.first, {'id': 1, 'title': 'Updated post'});
     });
 
+    test('Upsert with single onConflict column, not id', () async {
+      final data = {'user_id': 1, 'name': 'John Doe'};
+      const table = 'users';
+      // Insert a record
+      await mockSupabase.from(table).insert(data);
+      final users = await mockSupabase.from(table).select();
+      expect(users.first, data);
+
+      final updatedData = {
+        ...data,
+        'name': 'James Bond',
+      };
+      await mockSupabase.from(table).upsert(
+            updatedData,
+            onConflict: 'user_id',
+          );
+      final usersAfterUpdate =
+          await mockSupabase.from(table).select().eq('user_id', 1);
+      expect(usersAfterUpdate.length, 1);
+      expect(usersAfterUpdate.first, updatedData);
+    });
+
+    test('Upsert with multiple onConflict columns', () async {
+      final data = {'user': 2, 'post': 1, 'title': 'Initial post'};
+      // Test upserting a record
+      await mockSupabase.from('posts').insert(data);
+      final posts = await mockSupabase.from('posts').select();
+      expect(posts.first, data);
+
+      final updatedData = {
+        ...data,
+        'title': 'Updated post',
+      };
+      await mockSupabase.from('posts').upsert(
+            updatedData,
+            onConflict: 'user, post',
+          );
+      final postsAfterUpdate =
+          await mockSupabase.from('posts').select().eq('user', 2).eq('post', 1);
+      expect(postsAfterUpdate.length, 1);
+      expect(postsAfterUpdate.first, updatedData);
+    });
+
     test('Upsert then select', () async {
       // Test upserting a record
       await mockSupabase