Support arity 3 cast functions (#166)

elixir-ecto/ecto#4178 added support for using
an arity three cast function. This commit adds support to phoenix_ecto
as well.

lib/phoenix_ecto/html.ex

@@ -51,7 +51,7 @@ if Code.ensure_loaded?(Phoenix.HTML) do
 
           for changeset <- skip_replaced(changesets) do
             %{data: data, params: params} =
-              changeset = to_changeset(changeset, parent_action, module, cast)
+              changeset = to_changeset(changeset, parent_action, module, cast, nil)
 
             %Phoenix.HTML.Form{
               source: changeset,
@@ -82,7 +82,7 @@ if Code.ensure_loaded?(Phoenix.HTML) do
 
           for {changeset, index} <- Enum.with_index(changesets) do
             %{data: data, params: params} =
-              changeset = to_changeset(changeset, parent_action, module, cast)
+              changeset = to_changeset(changeset, parent_action, module, cast, index)
 
             index_string = Integer.to_string(index)
 
@@ -233,17 +233,20 @@ if Code.ensure_loaded?(Phoenix.HTML) do
       end
     end
 
-    defp to_changeset(%Ecto.Changeset{} = changeset, parent_action, _module, _cast),
+    defp to_changeset(%Ecto.Changeset{} = changeset, parent_action, _module, _cast, _index),
       do: apply_action(changeset, parent_action)
 
-    defp to_changeset(%{} = data, parent_action, _module, cast) when is_function(cast, 2),
+    defp to_changeset(%{} = data, parent_action, _module, cast, _index) when is_function(cast, 2),
       do: apply_action(cast!(cast, data), parent_action)
 
-    defp to_changeset(%{} = data, parent_action, _module, {module, func, arguments} = mfa)
+    defp to_changeset(%{} = data, parent_action, _module, cast, index) when is_function(cast, 3),
+      do: apply_action(cast!(cast, data, index), parent_action)
+
+    defp to_changeset(%{} = data, parent_action, _module, {module, func, arguments} = mfa, _imdex)
          when is_atom(module) and is_atom(func) and is_list(arguments),
          do: apply_action(apply!(mfa, data), parent_action)
 
-    defp to_changeset(%{} = data, parent_action, _module, nil),
+    defp to_changeset(%{} = data, parent_action, _module, nil, _index),
       do: apply_action(Ecto.Changeset.change(data), parent_action)
 
     defp cast!(cast, data) do
@@ -257,6 +260,17 @@ if Code.ensure_loaded?(Phoenix.HTML) do
       end
     end
 
+    defp cast!(cast, data, index) do
+      case cast.(data, %{}, index) do
+        %Ecto.Changeset{} = changeset ->
+          changeset
+
+        other ->
+          raise "expected on_cast/3 callback #{inspect(cast)} to return an Ecto.Changeset, " <>
+                  "got: #{inspect(other)}"
+      end
+    end
+
     defp apply!({module, func, arguments}, data) do
       case apply(module, func, [data, %{} | arguments]) do
         %Ecto.Changeset{} = changeset ->

test/phoenix_ecto/inputs_for_test.exs

@@ -280,6 +280,26 @@ defmodule PhoenixEcto.InputsForTest do
                ~s(<input id="user_comments_1_body" name="user[comments][1][body]" type="text" value="data2">)
   end
 
+  test "has many: inputs/for/4 with custom changeset with arity 3" do
+    changeset =
+      %User{comments: [%Comment{body: "data1"}, %Comment{body: "data2"}]}
+      |> cast(%{}, ~w()a)
+      |> cast_assoc(:comments, with: &Comment.changeset_with_position/3)
+
+    contents =
+      safe_inputs_for(
+        changeset,
+        :comments,
+        fn f ->
+          number_input(f, :position)
+        end
+      )
+
+    assert contents ==
+             ~s(<input id="user_comments_0_position" name="user[comments][0][position]" type="number" value="0">) <>
+               ~s(<input id="user_comments_1_position" name="user[comments][1][position]" type="number" value="1">)
+  end
+
   ## inputs_for embeds one
 
   test "embeds one: inputs_for/4" do

test/test_helper.exs

@@ -20,6 +20,7 @@ defmodule Comment do
 
   schema "comments" do
     field :body
+    field :position, :integer
   end
 
   def changeset(comment, params) do
@@ -38,6 +39,11 @@ defmodule Comment do
     |> cast(params, ~w(body)a)
     |> validate_length(:body, min: required_length)
   end
+
+  def changeset_with_position(comment, params, index) do
+    changeset(comment, params)
+    |> Ecto.Changeset.put_change(:position, index)
+  end
 end
 
 defmodule User do