I am trying to implement some restrictions on my MongoDB server:

Two databases on my server should be restricted regarding delete/drop operations - only a special user account should be allowed to do so. All the other database should be totally unrestricted (of course excluding the admin database):

I tried to model this situation using two users:

```
       |   database A & B   |  all the other databases |
---------------------------------------------------------
user a |    read & write    |       read & write       |
user b |      read-only     |       read & write       |

```

Making everybody read all databases is easy using the `readAnyDatabase`role.
However modelling that user b can only read database A & B but has read & write access to all the other databases (including those databases that are created later on) gives me a headache. 

How can this security model be implemented in MongoDB?