Suppose I have:

• Customer has_and_belongs_to_many reservations
• Reservation has_and_belongs_to_many customers

If I add a relation such as Customer.reservations << res, both the Customers and Reservations collections in my database will be updated. The updates to both collections should be wrapped in a single transaction, to ensure integrity of both collections.

This ticket would only apply if I am using a server version which supports multi-collection transactions. I forget exactly but I think this was introduced in 4.2 for non-shards and 4.4 for shards.

Transactions should not be used in the case of inverse_of: nil is used, because the relationship then becomes one-sided.