fix(core): preserve repeated default project selection

src/basic_memory/repository/project_repository.py

@@ -168,19 +168,24 @@ async def set_as_default(self, session: AsyncSession, project_id: int) -> Option
         Returns:
             The updated project if found, None otherwise
         """
-        # First, clear the default flag for all projects using direct SQL
+        target_project = await self.select_by_id(session, project_id)
+        if not target_project:
+            return None  # pragma: no cover
+
+        # Preserve the target row while clearing previous defaults. Clearing an already-default
+        # target through bulk SQL leaves its ORM identity-map value at True, so assigning True
+        # again emits no UPDATE and otherwise persists a database with no default project.
         await session.execute(
-            text("UPDATE project SET is_default = NULL WHERE is_default IS NOT NULL")
+            text(
+                "UPDATE project SET is_default = NULL "
+                "WHERE id != :project_id AND is_default IS NOT NULL"
+            ),
+            {"project_id": project_id},
         )
-        await session.flush()
 
-        # Set the new default project
-        target_project = await self.select_by_id(session, project_id)
-        if target_project:
-            target_project.is_default = True
-            await session.flush()
-            return target_project
-        return None  # pragma: no cover
+        target_project.is_default = True
+        await session.flush()
+        return target_project
 
     @override
     async def delete(self, session: AsyncSession, entity_id: int) -> bool:

tests/repository/test_project_repository.py

@@ -272,6 +272,27 @@ async def test_set_as_default(
         assert project2_updated.is_default is True
 
 
+@pytest.mark.asyncio
+async def test_set_as_default_keeps_existing_default(
+    project_repository: ProjectRepository, test_project: Project, session_maker
+):
+    """Setting the existing default again must leave it persisted as the default."""
+    async with db.scoped_session(session_maker) as session:
+        existing_default = await project_repository.find_by_id(session, test_project.id)
+        assert existing_default is not None
+        assert existing_default.is_default is True
+
+        updated_default = await project_repository.set_as_default(session, existing_default.id)
+        assert updated_default is not None
+        assert updated_default.is_default is True
+
+    # Verify from a new identity map so the assertion reflects persisted database state.
+    async with db.scoped_session(session_maker) as session:
+        persisted_default = await project_repository.get_default_project(session)
+        assert persisted_default is not None
+        assert persisted_default.id == test_project.id
+
+
 @pytest.mark.asyncio
 async def test_update_project(
     project_repository: ProjectRepository, sample_project: Project, session_maker