A mongoc_client_t object can include a user name and password that can be used to authenticate all the databases for that client. The current API, however, does not allow a database from this client to be authenticated for any user other than the one stored in the client. Two separate databases from this client cannot be authenticated for different users.

This was not a limitation of the 0.8x version of the MongoDB C driver. The function mongo_cmd_authenticate() was used to authenticate a given database for a given user, and not all databases had to be authenticated for the same user. We need to restore this ability to the 1.0.x C driver.