Add SetUserSessionIdPlug.

lib/pleroma/plugs/set_user_session_id_plug.ex

@@ -0,0 +1,15 @@
+defmodule Pleroma.Plugs.SetUserSessionIdPlug do
+  import Plug.Conn
+  alias Pleroma.User
+
+  def init(opts) do
+    opts
+  end
+
+  def call(%{assigns: %{user: %User{id: id}}} = conn, _) do
+    conn
+    |> put_session(:user_id, id)
+  end
+
+  def call(conn, _), do: conn
+end