fix: PostgreSQL cannot use get_chat_ids_by_model_id

Co-Authored-By: EntropyYue <164553692+EntropyYue@users.noreply.github.com>

backend/open_webui/models/chat_messages.py

@@ -15,6 +15,7 @@ from sqlalchemy import (
     Text,
     JSON,
     Index,
+    func,
 )
 
 ####################
@@ -279,25 +280,26 @@ class ChatMessageTable:
         db: Optional[Session] = None,
     ) -> list[str]:
         """Get distinct chat_ids that used a specific model."""
-        from sqlalchemy import distinct
 
         with get_db_context(db) as db:
-            query = db.query(distinct(ChatMessage.chat_id)).filter(
-                ChatMessage.model_id == model_id
-            )
+            query = db.query(
+                ChatMessage.chat_id,
+                func.max(ChatMessage.created_at).label("last_message_at"),
+            ).filter(ChatMessage.model_id == model_id)
             if start_date:
                 query = query.filter(ChatMessage.created_at >= start_date)
             if end_date:
                 query = query.filter(ChatMessage.created_at <= end_date)
 
-            # Order by most recent message in each chat
+            # Group by chat_id and order by most recent message in each chat
             chat_ids = (
-                query.order_by(ChatMessage.created_at.desc())
+                query.group_by(ChatMessage.chat_id)
+                .order_by(func.max(ChatMessage.created_at).desc())
                 .offset(skip)
                 .limit(limit)
                 .all()
             )
-            return [chat_id for (chat_id,) in chat_ids]
+            return [chat_id for chat_id, _ in chat_ids]
 
     def delete_messages_by_chat_id(
         self, chat_id: str, db: Optional[Session] = None