没有中间表的多对多 - 是否可能？

I have two entities that usually have one-to-many relationship, but in rare cases should have an opportunity to be many-to-many. I don't want to join the tables with the intermediate table for every query - and i guess there are preferable patterns for "rare many-to-many", - (perhaps with additional table for m-t-m, with duplicate records or something). Any ideas?I have two entities that usually have one-to-ma