Function: Update the engagements of a topic.

Function: Get a user’s topic engagements

Function: Check if a user is engaged in a topic or not

Function: Add a topic to user’s engagements

Function: Remove a topic from user’s engagements

Function: Get the query part responsible for JOINing objects to user IDs

Function: Get the users who have engaged in a topic

Function: Return the users who have engaged in a topic, directly with a database query

Function: Remove an object from all users

Function: Remove all users from all objects