fix: address performance and code quality issues in conversation history

Performance improvements:
- Fix N+1 query in get_conversations() using COUNT subquery instead of
  len(c.messages) which triggered lazy loading for each conversation
- Add SQLAlchemy engine caching to avoid creating new database connections
  on every request
- Add React.memo to ChatMessage component to prevent unnecessary re-renders
  during message streaming
- Move BOLD_REGEX to module scope to avoid recreating on each render

Code quality improvements:
- Remove 10+ console.log debug statements from AssistantChat.tsx and
  AssistantPanel.tsx that were left from development
- Add user feedback for delete errors in ConversationHistory - dialog now
  stays open and shows error message instead of silently failing
- Update ConfirmDialog to accept ReactNode for message prop to support
  rich error content

These changes address issues identified in the code review of PR #74
(conversation history feature).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/services/assistant_database.py

@@ -11,13 +11,17 @@ from datetime import datetime, timezone
 from pathlib import Path
 from typing import Optional
 
-from sqlalchemy import Column, DateTime, ForeignKey, Integer, String, Text, create_engine
+from sqlalchemy import Column, DateTime, ForeignKey, Integer, String, Text, create_engine, func
 from sqlalchemy.orm import declarative_base, relationship, sessionmaker
 
 logger = logging.getLogger(__name__)
 
 Base = declarative_base()
 
+# Engine cache to avoid creating new engines for each request
+# Key: project directory path (as posix string), Value: SQLAlchemy engine
+_engine_cache: dict[str, object] = {}
+
 
 def _utc_now() -> datetime:
     """Return current UTC time. Replacement for deprecated datetime.utcnow()."""
@@ -56,13 +60,23 @@ def get_db_path(project_dir: Path) -> Path:
 
 
 def get_engine(project_dir: Path):
-    """Get or create a SQLAlchemy engine for a project's assistant database."""
-    db_path = get_db_path(project_dir)
-    # Use as_posix() for cross-platform compatibility with SQLite connection strings
-    db_url = f"sqlite:///{db_path.as_posix()}"
-    engine = create_engine(db_url, echo=False)
-    Base.metadata.create_all(engine)
-    return engine
+    """Get or create a SQLAlchemy engine for a project's assistant database.
+
+    Uses a cache to avoid creating new engines for each request, which improves
+    performance by reusing database connections.
+    """
+    cache_key = project_dir.as_posix()
+
+    if cache_key not in _engine_cache:
+        db_path = get_db_path(project_dir)
+        # Use as_posix() for cross-platform compatibility with SQLite connection strings
+        db_url = f"sqlite:///{db_path.as_posix()}"
+        engine = create_engine(db_url, echo=False)
+        Base.metadata.create_all(engine)
+        _engine_cache[cache_key] = engine
+        logger.debug(f"Created new database engine for {cache_key}")
+
+    return _engine_cache[cache_key]
 
 
 def get_session(project_dir: Path):
@@ -94,23 +108,44 @@ def create_conversation(project_dir: Path, project_name: str, title: Optional[st
 
 
 def get_conversations(project_dir: Path, project_name: str) -> list[dict]:
-    """Get all conversations for a project with message counts."""
+    """Get all conversations for a project with message counts.
+
+    Uses a subquery for message_count to avoid N+1 query problem.
+    """
     session = get_session(project_dir)
     try:
+        # Subquery to count messages per conversation (avoids N+1 query)
+        message_count_subquery = (
+            session.query(
+                ConversationMessage.conversation_id,
+                func.count(ConversationMessage.id).label("message_count")
+            )
+            .group_by(ConversationMessage.conversation_id)
+            .subquery()
+        )
+
+        # Join conversation with message counts
         conversations = (
-            session.query(Conversation)
+            session.query(
+                Conversation,
+                func.coalesce(message_count_subquery.c.message_count, 0).label("message_count")
+            )
+            .outerjoin(
+                message_count_subquery,
+                Conversation.id == message_count_subquery.c.conversation_id
+            )
             .filter(Conversation.project_name == project_name)
             .order_by(Conversation.updated_at.desc())
             .all()
         )
         return [
             {
-                "id": c.id,
-                "project_name": c.project_name,
-                "title": c.title,
-                "created_at": c.created_at.isoformat() if c.created_at else None,
-                "updated_at": c.updated_at.isoformat() if c.updated_at else None,
-                "message_count": len(c.messages),
+                "id": c.Conversation.id,
+                "project_name": c.Conversation.project_name,
+                "title": c.Conversation.title,
+                "created_at": c.Conversation.created_at.isoformat() if c.Conversation.created_at else None,
+                "updated_at": c.Conversation.updated_at.isoformat() if c.Conversation.updated_at else None,
+                "message_count": c.message_count,
             }
             for c in conversations
         ]

ui/src/components/AssistantChat.tsx

@@ -78,22 +78,13 @@ export function AssistantChat({
 
   // Start or resume the chat session when component mounts or conversationId changes
   useEffect(() => {
-    console.log('[AssistantChat] useEffect running:', {
-      conversationId,
-      isLoadingConversation,
-      lastRef: lastConversationIdRef.current,
-      hasStarted: hasStartedRef.current
-    })
-
     // Skip if we're loading conversation details
     if (isLoadingConversation) {
-      console.log('[AssistantChat] Skipping - loading conversation')
       return
     }
 
     // Only start if conversationId has actually changed
     if (lastConversationIdRef.current === conversationId && hasStartedRef.current) {
-      console.log('[AssistantChat] Skipping - same conversationId')
       return
     }
 
@@ -101,23 +92,15 @@ export function AssistantChat({
     const isSwitching = lastConversationIdRef.current !== undefined &&
                         lastConversationIdRef.current !== conversationId
 
-    console.log('[AssistantChat] Processing conversation change:', {
-      from: lastConversationIdRef.current,
-      to: conversationId,
-      isSwitching
-    })
-
     lastConversationIdRef.current = conversationId
     hasStartedRef.current = true
 
     // Clear existing messages when switching conversations
     if (isSwitching) {
-      console.log('[AssistantChat] Clearing messages for conversation switch')
       clearMessages()
     }
 
     // Start the session with the conversation ID (or null for new)
-    console.log('[AssistantChat] Starting session with conversationId:', conversationId)
     start(conversationId)
   }, [conversationId, isLoadingConversation, start, clearMessages])
 
@@ -129,7 +112,6 @@ export function AssistantChat({
 
   // Handle selecting a conversation from history
   const handleSelectConversation = useCallback((id: number) => {
-    console.log('[AssistantChat] handleSelectConversation called with id:', id)
     setShowHistory(false)
     onSelectConversation?.(id)
   }, [onSelectConversation])

ui/src/components/AssistantPanel.tsx

@@ -62,13 +62,6 @@ export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelP
     timestamp: msg.timestamp ? new Date(msg.timestamp) : new Date(),
   }))
 
-  console.log('[AssistantPanel] State:', {
-    conversationId,
-    isLoadingConversation,
-    conversationDetailId: conversationDetail?.id,
-    initialMessagesCount: initialMessages?.length ?? 0
-  })
-
   // Persist conversation ID changes to localStorage
   useEffect(() => {
     setStoredConversationId(projectName, conversationId)
@@ -86,7 +79,6 @@ export function AssistantPanel({ projectName, isOpen, onClose }: AssistantPanelP
 
   // Handle selecting a conversation from history
   const handleSelectConversation = useCallback((id: number) => {
-    console.log('[AssistantPanel] handleSelectConversation called with id:', id)
     setConversationId(id)
   }, [])
 

ui/src/components/ChatMessage.tsx

@@ -5,6 +5,7 @@
  * Supports user, assistant, and system messages with neobrutalism styling.
  */
 
+import { memo } from 'react'
 import { Bot, User, Info } from 'lucide-react'
 import type { ChatMessage as ChatMessageType } from '../lib/types'
 
@@ -12,7 +13,10 @@ interface ChatMessageProps {
   message: ChatMessageType
 }
 
-export function ChatMessage({ message }: ChatMessageProps) {
+// Module-level regex to avoid recreating on each render
+const BOLD_REGEX = /\*\*(.*?)\*\*/g
+
+export const ChatMessage = memo(function ChatMessage({ message }: ChatMessageProps) {
   const { role, content, attachments, timestamp, isStreaming } = message
 
   // Format timestamp
@@ -112,13 +116,13 @@ export function ChatMessage({ message }: ChatMessageProps) {
             {content && (
               <div className={`whitespace-pre-wrap text-sm leading-relaxed ${config.textColor}`}>
                 {content.split('\n').map((line, i) => {
-                  // Bold text
-                  const boldRegex = /\*\*(.*?)\*\*/g
+                  // Bold text - use module-level regex, reset lastIndex for each line
+                  BOLD_REGEX.lastIndex = 0
                   const parts = []
                   let lastIndex = 0
                   let match
 
-                  while ((match = boldRegex.exec(line)) !== null) {
+                  while ((match = BOLD_REGEX.exec(line)) !== null) {
                     if (match.index > lastIndex) {
                       parts.push(line.slice(lastIndex, match.index))
                     }
@@ -196,4 +200,4 @@ export function ChatMessage({ message }: ChatMessageProps) {
       </div>
     </div>
   )
-}
+})

ui/src/components/ConfirmDialog.tsx

@@ -5,12 +5,13 @@
  * Used to confirm destructive actions like deleting projects.
  */
 
+import type { ReactNode } from 'react'
 import { AlertTriangle, X } from 'lucide-react'
 
 interface ConfirmDialogProps {
   isOpen: boolean
   title: string
-  message: string
+  message: ReactNode
   confirmLabel?: string
   cancelLabel?: string
   variant?: 'danger' | 'warning'
@@ -75,9 +76,9 @@ export function ConfirmDialog({
 
         {/* Content */}
         <div className="p-6">
-          <p className="text-[var(--color-neo-text-secondary)] mb-6">
+          <div className="text-[var(--color-neo-text-secondary)] mb-6">
             {message}
-          </p>
+          </div>
 
           {/* Actions */}
           <div className="flex justify-end gap-3">

ui/src/components/ConversationHistory.tsx

@@ -6,7 +6,7 @@
  */
 
 import { useState, useEffect } from 'react'
-import { MessageSquare, Trash2, Loader2 } from 'lucide-react'
+import { MessageSquare, Trash2, Loader2, AlertCircle } from 'lucide-react'
 import { useConversations, useDeleteConversation } from '../hooks/useConversations'
 import { ConfirmDialog } from './ConfirmDialog'
 import type { AssistantConversation } from '../lib/types'
@@ -50,10 +50,18 @@ export function ConversationHistory({
   onSelectConversation,
 }: ConversationHistoryProps) {
   const [conversationToDelete, setConversationToDelete] = useState<AssistantConversation | null>(null)
+  const [deleteError, setDeleteError] = useState<string | null>(null)
 
   const { data: conversations, isLoading } = useConversations(projectName)
   const deleteConversation = useDeleteConversation(projectName)
 
+  // Clear error when dropdown closes
+  useEffect(() => {
+    if (!isOpen) {
+      setDeleteError(null)
+    }
+  }, [isOpen])
+
   const handleDeleteClick = (e: React.MouseEvent, conversation: AssistantConversation) => {
     e.stopPropagation()
     setConversationToDelete(conversation)
@@ -63,16 +71,18 @@ export function ConversationHistory({
     if (!conversationToDelete) return
 
     try {
+      setDeleteError(null)
       await deleteConversation.mutateAsync(conversationToDelete.id)
       setConversationToDelete(null)
-    } catch (error) {
-      console.error('Failed to delete conversation:', error)
-      setConversationToDelete(null)
+    } catch {
+      // Keep dialog open and show error to user
+      setDeleteError('Failed to delete conversation. Please try again.')
     }
   }
 
   const handleCancelDelete = () => {
     setConversationToDelete(null)
+    setDeleteError(null)
   }
 
   const handleSelectConversation = (conversationId: number) => {
@@ -183,7 +193,19 @@ export function ConversationHistory({
       <ConfirmDialog
         isOpen={conversationToDelete !== null}
         title="Delete Conversation"
-        message={`Are you sure you want to delete "${conversationToDelete?.title || 'this conversation'}"? This action cannot be undone.`}
+        message={
+          deleteError ? (
+            <div className="space-y-3">
+              <p>{`Are you sure you want to delete "${conversationToDelete?.title || 'this conversation'}"? This action cannot be undone.`}</p>
+              <div className="flex items-center gap-2 p-2 bg-[var(--color-neo-danger)]/10 border border-[var(--color-neo-danger)] rounded text-sm text-[var(--color-neo-danger)]">
+                <AlertCircle size={16} className="flex-shrink-0" />
+                <span>{deleteError}</span>
+              </div>
+            </div>
+          ) : (
+            `Are you sure you want to delete "${conversationToDelete?.title || 'this conversation'}"? This action cannot be undone.`
+          )
+        }
         confirmLabel="Delete"
         cancelLabel="Cancel"
         variant="danger"