let a legacy migration whose comment reads like a scope get a down (#11765)

* fix: let a legacy migration whose comment reads like a scope get a down

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* test: pin adding a down to a migration whose comment predates scopes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/datatable_migration_add_down.rs

@@ -174,3 +174,34 @@ async fn test_add_down_to_a_migration_whose_comment_predates_role_annotations(
     assert_eq!(resp.status(), 200, "{}", resp.text().await?);
     Ok(())
 }
+
+/// Likewise for `-- schemas`: the scope annotation is held only to up SQL being written, so a
+/// definition whose stored up opens with prose starting with the word still accepts its down.
+#[sqlx::test(fixtures("datatable_migrations_grants"))]
+async fn test_add_down_to_a_migration_whose_comment_predates_scope_annotations(
+    db: Pool<Postgres>,
+) -> anyhow::Result<()> {
+    initialize_tracing().await;
+    setup_applied_migration_without_down(&db).await?;
+    let code_up = format!("-- Schemas: changes for the widgets feature\n{CODE_UP}");
+    sqlx::query("UPDATE datatable_migrations SET code_up = $1 WHERE workspace_id = 'dtmig-ws'")
+        .bind(&code_up)
+        .execute(&db)
+        .await?;
+
+    let server = ApiServer::start(db.clone()).await?;
+    let resp = authed(reqwest::Client::new().post(format!(
+        "http://localhost:{}/api/w/dtmig-ws/workspaces/upsert_datatable_migration/main",
+        server.addr.port()
+    )))
+    .json(&json!({
+        "timestamp": VERSION,
+        "name": "create_widgets",
+        "code_up": code_up,
+        "code_down": CODE_DOWN,
+    }))
+    .send()
+    .await?;
+    assert_eq!(resp.status(), 200, "{}", resp.text().await?);
+    Ok(())
+}

backend/windmill-api-workspaces/src/datatable_migrations.rs

@@ -1741,7 +1741,16 @@ async fn upsert_datatable_migration(
     if unchanged {
         return Ok(upserted_message);
     }
-    let scope = parse_migration_schemas(&payload.code_up)?;
+    // Only up SQL being written is held to the annotation: a stored up that opens with prose
+    // starting with the word `schemas` predates scopes, and must still accept a down added to it.
+    let up_changed = existing
+        .as_ref()
+        .is_none_or(|existing| existing.code_up != payload.code_up);
+    let scope = if up_changed {
+        parse_migration_schemas(&payload.code_up)?
+    } else {
+        None
+    };
     ensure_may_edit_migration(
         &db,
         &w_id,