more complete fix for commission and gallery pagination error

lib/philomena_web/controllers/commission_controller.ex

@@ -5,6 +5,7 @@ defmodule PhilomenaWeb.CommissionController do
   alias Philomena.Repo
   import Ecto.Query
 
+  plug PhilomenaWeb.MapParameterPlug, [param: "commission"] when action in [:index]
   plug :preload_commission
 
   def index(conn, params) do
@@ -12,10 +13,6 @@ defmodule PhilomenaWeb.CommissionController do
       commission_search(params["commission"])
       |> Repo.paginate(conn.assigns.scrivener)
 
-    # Scrub parameters to avoid form error...
-    params = Map.put(conn.params, "commission", permit_map(conn.params["commission"]))
-    conn = Map.put(conn, :params, params)
-
     render(conn, "index.html",
       title: "Commissions",
       commissions: commissions,
@@ -88,9 +85,6 @@ defmodule PhilomenaWeb.CommissionController do
   defp presence(object),
     do: object
 
-  defp permit_map(x) when is_map(x), do: x
-  defp permit_map(_), do: nil
-
   defp to_f(input) do
     case Float.parse(to_string(input)) do
       {float, _rest} -> float

lib/philomena_web/controllers/gallery_controller.ex

@@ -10,6 +10,7 @@ defmodule PhilomenaWeb.GalleryController do
   import Ecto.Query
 
   plug PhilomenaWeb.FilterBannedUsersPlug when action in [:new, :create, :edit, :update, :delete]
+  plug PhilomenaWeb.MapParameterPlug, [param: "gallery"] when action in [:index]
 
   plug :load_and_authorize_resource,
     model: Gallery,

lib/philomena_web/plugs/map_parameter_plug.ex

@@ -0,0 +1,39 @@
+defmodule PhilomenaWeb.MapParameterPlug do
+  # A bunch of crappy behaviors all interacting to create a
+  # symphony of failure:
+  #
+  # 1.) Router helpers do not strip nil query parameters.
+  #     iex> Routes.gallery_path(conn, :index, gallery: nil)
+  #     "/galleries?gallery="
+  #
+  # 2.) Pagination always sets the parameter in the route in order
+  #     to preserve the query across multiple pages
+  #
+  # 3.) When received by the router, an empty param is treated as
+  #     an empty string instead of nil
+  #
+  # 4.) Phoenix.HTML.Form.form_for/2 raises an error if you try to
+  #     use it on a conn object which is a string instead of a map
+  #     (or nil)
+
+  @spec init(Keyword.t()) :: Keyword.t()
+  def init(opts) do
+    opts
+  end
+
+  @spec call(Plug.Conn.t(), Keyword.t()) :: Plug.Conn.t()
+  def call(conn, opts) do
+    param = Keyword.fetch!(opts, :param)
+    value = conn.params[param]
+
+    cond do
+      is_map(value) ->
+        conn
+
+      true ->
+        params = Map.delete(conn.params, param)
+
+        Map.put(conn, :params, params)
+    end
+  end
+end