Question

If I want to maintain an ordered list in Firebase, it seems like the best way to do it is to manually assign a priority to each item in my list. That means if I insert or remove an item from the list, I have to update the priorities of all the items following it. For an item at the beginning of the list, this means updating every item in the list. Is there a better performing data structure or algorithm to use in this case?

Answer 1

You can create an ordered list by setting the priority of elements appropriately. Items in a list are ordered lexigraphically by priority, or if the priority can be parsed to a number, by numeric value.

If you want to insert items into the middle of an existing list, modifying the priorities of the existing items would work, but would be horribly inefficient. A better approach is just to pick a priority between the two items where you want to insert the value and set that priority for the new item.

For example, if you had element 1 with priority “a”, and element 2 with priority “b”, you could insert element 3 between the two with priority “aa” (or “aq”, “az”, etc).

In our experience, most times when you create an ordered list, you don’t necessarily know the position in the list you want to insert the item beforehand. For example, if you’re creating a Leader Board for a game, you don’t know in advance that you want to place a new score 3rd in the list, rather you know you want to insert it at whatever position score 10000 gets you (which might happen to be third). In this case, simply setting the priority to the score will accomplish this. See our Leader Board example here:
https://www.firebase.com/tutorial/#example-leaderboard