Set roles in mongo.add.user

[stanstrup]

It would be nice if it was possible to define the "roles" with mongo.add.user.

[dselivanov]

Unfortunately, underlying version of mongo-c-driver doesn't support this functionality. But we can do this using mongo.command and this createUser command. For example mongo.aggregation and some other functions are coded this way.
Pull request is very welcome! :-)