Here is how I solved it in my app:

1. I store the params in handle_params as part of the assigns:

    def handle_params(params, url, socket) do
      # You could store the params as is but I like to store
      # only what has been parsed/validated.
      params = parse_params(params)
      {:noreply, assign(socket, :params, params)
    end
   

2. Create a helper called self_path:

    def self_path(socket, action, extra) do
      Routes.live_path(socket, __MODULE__, action, Enum.into(extra, socket.assign.params))
    end
   

3. Now I do:

    push_path(socket, to: self_path(socket, :search, %{"sort_by" => "foo"}))
   

FWIW, you can also directly use the URI rather than calling the Routes helpers. The above code does this as well, but I wanted to provide a more concise version if you don’t need as many bells and whistles.

  def handle_params(_params, uri, socket) do
    {:noreply, assign(socket, :uri, URI.parse(uri))}
  end

  def handle_event("prev_page", _, socket = %{assigns: %{pager: %{page: page}, uri: uri}}) do
    current_params = URI.decode_query(uri.query || "")
    new_params = Map.put(current_params, "page", page - 1)
    to = uri.path <> "?" <> URI.encode_query(new_params)

    {:noreply, push_patch(socket, to: to)}
  end

Worth adding that instead of URI.decode_query/1 and URI.encode_query/1, you might want to use more robust Plug.Conn.Query.decode/1 and Plug.Conn.Query.encode/1, which do better encoding/decoding of lists in URL (e.g. ?ids[]=1&ids[]=2).

I.e.

URI.decode_query("ids[]=1&ids[]=2")
# => %{"ids[]" => "2"}

Plug.Conn.Query.decode("ids[]=1&ids[]=2")
# => %{"ids" => ["1", "2"]}