But manager is not a relational database, it’s an object-oriented or document database. And this is why many request aren’t really feasible to implement like many-to-many relations, multi-dimensions, SQL queries, etc. It’s not a bad thing though, it is a trade-off after all.
Here here 
That must be why it’s impossible to store a single object inside two other objects without a strict hierarchy in place: one has to be the parent of another. This is equivalent to having the Accounts receivable be the parent of Accounts payable or the other way round which doesn’t make any sense.
I tried to research this and the more I learn about OODB and it’s advantages, the more I appreciate the trade-offs that @lubos made. For example you will not easily find another software with a magical Undo button like manager and that is because while relational databases keep logs, manager has document versioning which is way better. Anyway … I could be speaking nonsense for all I know. @lubos needs to confirm this.
That’s exactly what I meant by “no offsetting rules”. So actually @Davide, you and I are in agreement. 