In Datomic, you can find ways to make one-to-many associations between two entities, but what about an ordered one-to-many association?

One of the most basic data structures for ordered collections is an array.

(def friend-ids [4 3 8])

However, Datomic unfortunately doesn’t store array types. These get turned into sets, which are unordered and only hold unique values.

Another option is using a linked list which, as I talk about in another post, is more complicated and has a bit more overhead that comes with it.

This leaves us with a stringified vector.

(def friends "[4 3 8]")

While this may not be the most efficient data structure as far as memory and performance, it’s simple to implement, and therefore less prone to bugs.

However, with a stringified vector, we lose referential integrity. This means that we cannot guarantee that every ID in the vector actually exists, so we’d need to account for nil cases when working with the items in the collection.

Still, I think if the Linked List can be hidden behind a stable list interface that doesn’t live on the entities themselves but in its own entity, this would probably be the best option.