fix: resolve chat model selection and session display issues

- Add nullable_fields support to ObjectModel base class
- Configure ChatSession to allow model_override to be cleared to null
- Fix JSX conditional that rendered "0" when message_count was 0
- Display model name instead of raw ID in session manager

Fixes issues:
1. Switching to default model now persists correctly
2. Session list shows human-readable model names
3. Sessions with 0 messages no longer show "0" badge

frontend/src/components/source/SessionManager.tsx

@@ -1,16 +1,16 @@
 'use client'
 
-import { useState } from 'react'
+import { useState, useMemo } from 'react'
 import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
 import { Input } from '@/components/ui/input'
 import { ScrollArea } from '@/components/ui/scroll-area'
 import { Badge } from '@/components/ui/badge'
-import { 
-  MessageSquare, 
-  Plus, 
-  Trash2, 
-  Edit2, 
+import {
+  MessageSquare,
+  Plus,
+  Trash2,
+  Edit2,
   Check,
   X,
   Clock
@@ -27,6 +27,7 @@ import {
   AlertDialogTitle,
 } from '@/components/ui/alert-dialog'
 import { BaseChatSession } from '@/lib/types/api'
+import { useModels } from '@/lib/hooks/use-models'
 
 interface SessionManagerProps {
   sessions: BaseChatSession[]
@@ -53,6 +54,16 @@ export function SessionManager({
   const [editTitle, setEditTitle] = useState('')
   const [deleteConfirmId, setDeleteConfirmId] = useState<string | null>(null)
 
+  const { data: models } = useModels()
+
+  // Helper to get model name from ID
+  const getModelName = useMemo(() => {
+    return (modelId: string) => {
+      const model = models?.find(m => m.id === modelId)
+      return model?.name || 'Custom Model'
+    }
+  }, [models])
+
   const handleCreateSession = () => {
     if (newSessionTitle.trim()) {
       onCreateSession(newSessionTitle.trim())
@@ -211,14 +222,14 @@ export function SessionManager({
                           <Clock className="h-3 w-3" />
                           {formatDistanceToNow(new Date(session.created), { addSuffix: true })}
                         </div>
-                        {session.message_count && session.message_count > 0 && (
+                        {session.message_count != null && session.message_count > 0 && (
                           <Badge variant="secondary" className="mt-2 text-xs">
                             {session.message_count} messages
                           </Badge>
                         )}
                         {session.model_override && (
                           <Badge variant="outline" className="mt-2 ml-2 text-xs">
-                            {session.model_override}
+                            {getModelName(session.model_override)}
                           </Badge>
                         )}
                       </>

open_notebook/domain/base.py

@@ -25,6 +25,7 @@ T = TypeVar("T", bound="ObjectModel")
 class ObjectModel(BaseModel):
     id: Optional[str] = None
     table_name: ClassVar[str] = ""
+    nullable_fields: ClassVar[set[str]] = set()  # Fields that can be saved as None
     created: Optional[datetime] = None
     updated: Optional[datetime] = None
 
@@ -167,7 +168,11 @@ class ObjectModel(BaseModel):
 
     def _prepare_save_data(self) -> Dict[str, Any]:
         data = self.model_dump()
-        return {key: value for key, value in data.items() if value is not None}
+        return {
+            key: value
+            for key, value in data.items()
+            if value is not None or key in self.__class__.nullable_fields
+        }
 
     async def delete(self) -> bool:
         if self.id is None:

open_notebook/domain/notebook.py

@@ -389,6 +389,7 @@ class Note(ObjectModel):
 
 class ChatSession(ObjectModel):
     table_name: ClassVar[str] = "chat_session"
+    nullable_fields: ClassVar[set[str]] = {"model_override"}
     title: Optional[str] = None
     model_override: Optional[str] = None