I don’t think there’s any problem in using pattern matching in controller methods. I’ve done this in one of my projects myself and it works great:

  def insert_attachment(conn, %{"ticket_message" => ticket_message,
                                "ticket_id"      => ticket_id}) do

    ticket = Repo.get(Ticket, ticket_id)
    upload = Map.get(ticket_message, "file")
    TicketAttachmentController.insert(conn, ticket, upload)
  end

  def insert_attachment(conn, %{"ticket_id"      => ticket_id}) do

    conn
    |> put_flash(:error, "No file was selected.")
    |> redirect(to: ticket_path(conn, :show, ticket_id))
  end

Your match does only “fire” when you have all the parameters "reqsrc", "count", and "first" available. In the requested “path” /gigaset?reqsrc=auto" there is only "reqsrc" available, so you need to add another clause after your current one, which needs to be like this:

def gigaset(conn, params = %{"reqsrc" => "auto"}), do: :stuff

Wait.. after testing it again it seems to not work as expected

Requesting /gigaset?reqsrc=auto yields a ‘no matching action clause to process request’ error on the definition of the first method. Is there a way to do this, at all?

I ended up rewriting into this long method (with some more changes):

  def gigaset(conn, params = %{ "reqsrc" => reqsrc, "count" => limit, "first" => offset }) do
    query = case reqsrc do
      # User quering phonebook from Gigaset base
      "user" ->
        name = String.replace(params["ln"], "*", "%")
        from e in Entry, where: ilike(e.name, ^name)

      # Caller ID from Gigaset base
      "auto" ->
        number = params["hm"]
        from e in Entry, where: e.work == ^number or e.home == ^number or e.mobile == ^number
    end

    # Total before pagination
    total = Repo.one(from e in query, select: count(e.id))

    # Paginated entries
    offset = String.to_integer(offset) - 1
    query = from e in query, limit: ^limit, offset: ^offset
    entries = Repo.all(query)

    render conn, entries: entries, total: total, offset: offset
  end

All feedback is welcome! I realise some of this belongs in the model probably.