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How to smartly keep a set of Django ORM objects in order for drag-n-drop like situations.
ManyToManyFields confuse a lot of people. The way you relate objects to each other is just different enough from dealing with ForeignKeys and just uncommon enough in day-to-day Django development that it's easy to forget all the little tricks for dealing with them.
A very common pattern in a Django Project is to have some kind of 'model-type' relationship, where you have some kind of object that can only belong to one of the types defined in the database.