I'd like to discuss this choice.

some thoughts on it:

- you definitely want to over-write the private attr accessing method?
- I'm concerned that there would be a case where `discussion.private == nil`. When would a discussion not have a privacy set?
- the competing architecture I'm thinking of is one that ensures private discussions if a group setting changes. 

do you need #private? or does rails fall back to #private if it can't find the boolean query?