RFC: `joinLists`

[Kelin2025]

Case

Sometimes you need one-to-many connection between two KVs

For example, there's "Swaps" widgets and you need to gather swaps lists for each widget

Solution

Add joinLists method

const widgetSwaps = joinLists({
  kv: Widget.model.$kv,
  join: Swap.model.$kv,
  fn: (widget, swap) => widget.swapIds.includes(swap.id),
  orderBy: (a, b) => a.timestamp - b.timestamp
})

[Kelin2025]

Actually I think the naming of this one is incorrect but wonder what would be a better name

[Minhir]

This could be solved by createMap/createSelection with source field and filter option (#17 (comment)). You can pass swapIds as source and use filter to check includes.
I think it looks more straightforward.

[Kelin2025]

The idea of joinLists and grabItems methods is to provide a formalized way to join KVs without manually iterating through them
The problem with solving these cases through createMap/createSelection is that you'll pass other KVs as a source and handle joining inside fn, so your app

• Will have knowledge about KV structure
• And it's a lot of boilerplate code too

[Minhir]

I agree that we need some utils which are working "key per key" like SQL joins does. I'm still not sure how to make it easier. We can start with current joinLists approach.