Optimize local folder file tree lazy loading

rust/crates/mnote-web/src/routes/kernel.rs

@@ -2,7 +2,8 @@ use crate::app::AppState;
 use crate::context::RequestContext;
 use crate::error::WebError;
 use crate::routes::local_folder_source::{
-    ensure_local_workspace_read_access_with_state, load_local_folder_file_tree_snapshot,
+    ensure_local_workspace_read_access_with_state, load_local_folder_file_tree_children_snapshot,
+    load_local_folder_file_tree_snapshot, load_local_folder_file_tree_snapshot_with_reveal,
     load_local_folder_page_tree_snapshot,
 };
 use crate::routes::query_support::resolve_effective_workspace_id;
@@ -28,6 +29,7 @@ pub struct KernelProjectionQuery {
     pub max_results: Option<usize>,
     pub source_kind: Option<String>,
     pub root_uri: Option<String>,
+    pub parent_relative_path: Option<String>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -88,7 +90,27 @@ async fn project_projection(
         ensure_local_workspace_read_access_with_state(&state, &context, root_uri)
             .map_err(|error| error.with_context(&context))?;
         let snapshot = if projection == KernelProjectionKind::FileTree {
-            load_local_folder_file_tree_snapshot(root_uri)?
+            if let Some(parent_relative_path) = query
+                .parent_relative_path
+                .as_deref()
+                .map(str::trim)
+                .filter(|value| !value.is_empty())
+            {
+                load_local_folder_file_tree_children_snapshot(root_uri, parent_relative_path)?
+            } else if query
+                .root_node_id
+                .as_deref()
+                .map(str::trim)
+                .filter(|value| !value.is_empty())
+                .is_some()
+            {
+                load_local_folder_file_tree_snapshot_with_reveal(
+                    root_uri,
+                    query.root_node_id.as_deref(),
+                )?
+            } else {
+                load_local_folder_file_tree_snapshot(root_uri)?
+            }
         } else {
             load_local_folder_page_tree_snapshot(root_uri)?
         };
@@ -371,6 +393,56 @@ mod tests {
         let _ = std::fs::remove_dir_all(&root);
     }
 
+    #[tokio::test]
+    async fn local_folder_file_projection_can_load_direct_children_by_parent_path() {
+        let root = std::env::temp_dir().join(format!(
+            "mnote-local-kernel-children-projection-{}",
+            std::process::id()
+        ));
+        let _ = std::fs::remove_dir_all(&root);
+        std::fs::create_dir_all(root.join("docs").join("nested")).expect("create nested dirs");
+        std::fs::write(root.join("docs").join("README.md"), "# README\n").expect("write readme");
+        std::fs::write(root.join("docs").join("nested").join("deep.md"), "# Deep\n")
+            .expect("write deep");
+        let root_uri = format!("file://{}", root.display());
+        initialize_local_workspace_for_actor("dev-user", &root_uri).expect("init local workspace");
+
+        let response = app()
+            .oneshot(
+                Request::builder()
+                    .uri(format!(
+                        "/api/tree/projections/file?workspaceId=local-ws:dev-user:my-space&sourceKind=local_folder&rootUri={root_uri}&parentRelativePath=docs"
+                    ))
+                    .header("x-mnote-actor-id", "dev-user")
+                    .header("x-mnote-actor-type", "user")
+                    .body(Body::empty())
+                    .expect("request"),
+            )
+            .await
+            .expect("response");
+
+        assert_eq!(response.status(), StatusCode::OK);
+        let body = to_bytes(response.into_body(), usize::MAX)
+            .await
+            .expect("body");
+        let payload: Value = serde_json::from_slice(&body).expect("json");
+        let items = payload["result"]["items"].as_array().expect("items");
+        let titles = items
+            .iter()
+            .filter_map(|item| item["title"].as_str())
+            .collect::<Vec<_>>();
+
+        assert_eq!(payload["result"]["parentRelativePath"], "docs");
+        assert!(titles.contains(&"README.md"));
+        assert!(titles.contains(&"nested"));
+        assert!(!titles.contains(&"deep.md"));
+        assert!(items
+            .iter()
+            .all(|item| item["parentNodeId"].as_str() == Some("local:node:docs")));
+
+        let _ = std::fs::remove_dir_all(&root);
+    }
+
     #[tokio::test]
     async fn tree_projection_routes_keep_ok_response_shape() {
         let response = app()