Hi Henning,
Thank you very much for your reply. I have a question: what do you mean when you talk about using ndb_mongodb module? Do you mean to use it from the configuration file or inside a module code? Our use case is to save all contacts related to an AOR in the same MongoDB document as an array in order to have all information inside a document and avoid the use of indexes. But, of course, we want to use the 'save' function (from registrar module) and not to launch either raw queries or ndb_mongodb commands, if possible.
Regards, Mario