An issue with many to many relationships, or any relationship that uses "shortcut" cannot be maintained properly. Need to build a screen and logic to allow the linking and creation of these relationships.
Simple double belongs to table, used with shortcut - standard many-to-many setup.
Double key with a few attributes, ie parent1id, parent2id, code - this table only has relevance as a link between the two parents, it is not an object "Entity" but a "Relationship" with extra identifiers.
An issue with many to many relationships, or any relationship that uses "shortcut" cannot be maintained properly. Need to build a screen and logic to allow the linking and creation of these relationships.