Android app shows ghost lastMessage after user deletion (room.lastMessage not cleaned up)

Description:

When a user is deleted from Rocket.Chat, all of their messages are removed from the rocketchat_message collection. However, if one of these messages was the last message in a room, the lastMessage field in the corresponding rocketchat_room document is not cleaned up.

This results in a ghost message that is only visible on the Android mobile client. The web and desktop clients ignore the dangling lastMessage pointer.

Steps to reproduce:

1. Create a room (e.g., #general).
2. Post a message in the room as User A.
3. Delete User A from the system.
   • The message disappears from the rocketchat_message collection.
   • But the room document (rocketchat_room) still has the message stored in its lastMessage field.
4. Open the room on the Android mobile app → the ghost message is shown.
5. Open the same room in the browser or desktop client → no ghost message is shown.

Expected behavior:

• When a user (and their messages) are deleted, the lastMessage field in rocketchat_room should be cleared or updated to point to the latest valid message.
• No ghost messages should appear in any client.

Actual behavior:

• Ghost message still appears in Android mobile client.
• The message is not in rocketchat_message anymore.
• The room’s lastMessage field still references it.
• Web and desktop clients do not display it, but Android does.

Server Setup Information:

• Version of Rocket.Chat Server: 7.10.0
• License Type: Community
• Number of Users: 20
• Operating System: Ubuntu 24.04 LTS
• Deployment Method: Docker
• Number of Running Instances: 1
• DB Replicaset Oplog: Disabled
• NodeJS Version: 22.16.0
• MongoDB Version: 6.0.13

Client Setup Information

• Desktop App or Browser Version: 4.9.0 / Firefox 142.0.1

• Operating System: Linux, macOS, Windows

• Mobile App: Android Rocket.Chat app (latest release from Play Store): 4.63.0.96963

Additional context

• The issue only affects mobile clients (tested on multiple Android devices).
• It seems the Android app uses the lastMessage field in rocketchat_room to render channel previews, while web/desktop clients query rocketchat_message directly.
• Manual workaround:

  db.rocketchat_room.updateOne(
    { _id: "GENERAL" },
    { $unset: { lastMessage: "" } }
  )
  

Trace the user-deletion flow that removes messages from the rocketchat_message collection and inspect how the related rocketchat_room document's lastMessage field is handled. Reproduce the deletion with a room's final message, then verify that the field is cleared or points to a valid remaining message and that the Android client no longer shows a ghost message.