akramic
Using Elixir as a Presence Client
I’m trying to follow the documentation on using Elixir as a Presence Client as outlined here: [Phoenix.Presence — Phoenix v1.8.8]
Channel join code :
@impl true
def join("online", params, socket) do
IO.inspect(params, label: "PARAMS on join")
send(self(), :after_join)
{:ok, socket}
end
@impl true
def handle_info(:after_join, socket) do
IO.inspect(socket)
user_id = socket.assigns.user
{:ok, _} = Presence.track(socket, user_id, %{
user_id: user_id,
online_at: inspect(System.system_time(:second))
})
push(socket, "presence_state", Presence.list(socket))
{:noreply, socket}
end
Code for presence.ex
def init(_opts) do
{:ok, %{}} # user-land state
end
def handle_metas(topic, %{joins: joins, leaves: leaves}, presences, state) do
IO.inspect(joins, label: "JOINS")
# fetch existing presence information for the joined users and broadcast the
# event to all subscribers
for {user_id, presence} <- joins do
user_data = %{user: presence.user, metas: Map.fetch!(presences, user_id)}
msg = {MyApp.PresenceClient, {:join, user_data}}
Phoenix.PubSub.local_broadcast(MyApp.PubSub, topic, msg)
end
# fetch existing presence information for the left users and broadcast the
# event to all subscribers
for {user_id, presence} <- leaves do
IO.inspect(joins, label: "LEAVES")
metas =
case Map.fetch(presences, user_id) do
{:ok, presence_metas} -> presence_metas
:error -> []
end
user_data = %{user: presence.user, metas: metas}
msg = {MyApp.PresenceClient, {:leave, user_data}}
Phoenix.PubSub.local_broadcast(MyApp.PubSub, topic, msg)
end
{:ok, state}
end
end
I’m able to join the channel but get the following error:
(KeyError) key :user not found in: %{metas: [%{user: 1, online_at: "1728611576", phx_ref: "F_1DEghkHFXOsARm"}]}
I think that the error is due to this line:
user_data = %{user: presence.user, metas: Map.fetch!(presences, user_id)}
I’d be grateful if someone could point me in the direction of resolving this error.
Many thanks and apologies if this duplicates another post - I couldn’t find anything related to this on the forum.
Most Liked
akramic
This is called because this line
`
`
should be
user_data = %{user: user_id, metas: Map.fetch!(presences, user_id)}
and
user_data = %{user: presence.user, metas: metas}
should be
user_data = %{user: user_id, metas: metas}
I wasn’t looking closely enough at the data structure for presences and presence.
Hope it helps someone.
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