Presence List/Fetch Override: Too many unnecessary database calls, 4+ calls per join, how to fix?

[peppy]

The two elixir snippets are pretty much all there is and comes from the very basic documentation examples. There were a couple of other unresolved concerns on the internet about this exact problem related to the fetch override and duplicate queries being called:

phoenix presence fetch/2 override

When is Phoenix Presence's fetch callback called?

Hi all From the docs, “The fetch/2 callback is triggered when using list/1 and serves as a mechanism to fetch presence information a single time, before broadcasting the information to all channel subscribers”. I added an inspect of Map.keys(entries) in the fetch callback, and it seems to be called twice when a new user is tracked, one time with an empty map and the second time with the full list of users in the presence. The client of this app already keeps the records in memo…

The rest of the non-elixir stuff below the presence.ex code is just my custom IO.inspect to show when a query is run and what the query looks like (to show which ids are trying to be fetched). I added that there just to show that there are indeed 4 queries being called for just one person joining the room (two queries are exact duplicates), and 2 queries for one person leaving the room.

The problem is, I don’t even know why or where this many queries are being called. I think it’s an unnecessary waste of database resources. It might even be a bug.