Salut Steeve! Yes, we have the middleware in our project and I’m able to call the api routes to get data from the desired collections. Actually I need to compare metadata in related collections and update data in the local collection by the user call by the running a script (Node + js). This script (Node + js) does not work properly and we do not use refs in our mongoose schema . I’d like ro rewrite it with using aggregation func. Could you, pls, write for me the short example of code how to create/merge those collections to temporary/ virtual DB, then put make to aggregation func.?