If someone has access to your database, you've already got more problems then someone knowing that author X is associated with post Y.
Any any sort of scheme that you come up with the change the author ID will not be great because it will be able to be found out, so if someone will find out that scheme anyway.
To be honest, I would not worry about this. There's basically no reason for anything like this because you shouldn't be allowing access to the database for people that could have issues with this in the first place. You'll have the relationship set up in the admin area anyway, so somewhere along the line there is something that tells how that's done, and someone will be able to find it.
Keep in mind that someone hacking into your admin account to see the actual relationships is a while lot more likely then someone hacking into the database and reading the raw data.