Phoenix.HTML.Form.form_for/4 is not working with Changeset

[eniolopes]

According to the documentation here, form_for/4 receives the Ecto.Changeset and converts it to a form, which is passed to the function as the argument f.

I'm following the Programming Phoenix book building the Rumbl application, when rendering the user form I'm getting the following error:

[error] #PID<0.372.0> running Rumbl.Endpoint terminated
Server: localhost:4000 (http)
Request: GET /users/new
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol Phoenix.HTML.FormData not implemented for %Ecto.Changeset{action: nil, changes: %{}, constraints: [], errors: [], filters: %{}, model: %Rumbl.User{__meta__: #Ecto.Schema.Metadata<:built>, id: nil, inserted_at: nil, name: nil, password: nil, password_hash: nil, updated_at: nil, username: nil}, optional: [], opts: [], params: nil, prepare: [], repo: nil, required: [:name, :username], types: %{id: :id, inserted_at: Ecto.DateTime, name: :string, password: :string, password_hash: :string, updated_at: Ecto.DateTime, username: :string}, valid?: false, validations: [username: {:length, [min: 1, max: 20]}]}
        (phoenix_html) lib/phoenix_html/form_data.ex:1: Phoenix.HTML.FormData.impl_for!/1
        (phoenix_html) lib/phoenix_html/form_data.ex:15: Phoenix.HTML.FormData.to_form/2
        (phoenix_html) lib/phoenix_html/form.ex:234: Phoenix.HTML.Form.form_for/4
        (rumbl) web/templates/user/new.html.eex:2: Rumbl.UserView."new.html"/1
        (rumbl) web/templates/layout/app.html.eex:29: Rumbl.LayoutView."app.html"/1
        (phoenix) lib/phoenix/view.ex:344: Phoenix.View.render_to_iodata/3
        (phoenix) lib/phoenix/controller.ex:633: Phoenix.Controller.do_render/4
        (rumbl) web/controllers/user_controller.ex:1: Rumbl.UserController.action/2
        (rumbl) web/controllers/user_controller.ex:1: Rumbl.UserController.phoenix_controller_pipeline/2
        (rumbl) lib/phoenix/router.ex:261: Rumbl.Router.dispatch/2
        (rumbl) web/router.ex:1: Rumbl.Router.do_call/2
        (rumbl) lib/rumbl/endpoint.ex:1: Rumbl.Endpoint.phoenix_pipeline/1
        (rumbl) lib/plug/debugger.ex:93: Rumbl.Endpoint."call (overridable 3)"/2
        (rumbl) lib/phoenix/endpoint/render_errors.ex:34: Rumbl.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

Not sure what I'm missing, I don't see a defimpl for Ecto.Changeset on Phoenix.HTML.FormData

I'm using Elixir 1.2.5 with Phoenix 1.1.5.

I'm probably missing something really simple.

[michalmuskala]

The phoenix_ecto package contains protocol definitions for ecto. Do you include it?

[eniolopes]

Yes.

* phoenix_ecto 2.0.2 (Hex package) (mix)
  locked at 2.0.2 (phoenix_ecto) 1aff51d1
  ok

[eniolopes]

@michalmuskala I don't see it being included in web/web.ex should it be?

[josevalim]

No, it should not be included. There is definitely something happening here. Does it work if you remove _build and compile again?

[eniolopes]

@josevalim It did, no idea why.

[josevalim]

We have improved such checks in Elixir v1.3.0 so, after you update, please let us know if it happens again!

[eniolopes]

Awesome! Thanks for the fast response. :)

[satb]

I ran into this same issue. Had no idea why it was happening until I stumbled into this issue...cleaned the _build and recompiled and everything was good again. Using elixir 1.3.4, ecto 2.0.6 and phoenix 1.2

[pdelanauze]

Elixir v1.4.2 , phoenix 1.2.4, ecto 2.1.4 (umbrella app setup), ran into the same issue, clearing out _build fixed it !

[7stud]

We have improved such checks in Elixir v1.3.0 so, after you update, please let us know if it happens again!

phoenix 1.4.0
elixir 1.6.6
phoenix_ecto 4.0.0 (dep: ecto 3.1.4)

I'm following along with the ebook Phoenix in Action (final edition), which creates an umbrella app, containing two apps: Auction and AuctionWeb. On p. 168 the book implements the new action in AuctionWeb.ItemController. The corresponding template, new.html.eex, employs the form_for() helper, which takes a changeset as the first argument:

<%= form_for @item, Routes.item_path(@conn, :create), fn f -> %>

@item comes from here:

  def new(conn, _params) do
    item = Auction.new_item()
    render(conn, "new.html", item: item)
  end

And here's Auction.new_item:

  def new_item() do
    Item.changeset(%Item{})
  end

And, here's Item.changeset():

defmodule Auction.Item do
  use Ecto.Schema
  import Ecto.Changeset

  schema "items" do
    field :title, :string
    field :description, :string
    field :ends_at, :utc_datetime
    timestamps()
  end

  def changeset(%Auction.Item{}=item, attrs \\ %{}) do
    item
    |> cast(attrs, [:title, :description, :ends_at])
    |> validate_required([:title])
    |> validate_length(:title, min: 3)
    |> validate_length(:description, max: 20)
    |> validate_change(:ends_at, &validate_in_future/2)
  end

  defp validate_in_future(:ends_at, date) do
    case DateTime.compare(date, DateTime.utc_now()) do
      :lt -> [ends_at: "must be in the future"]
      _   -> []
    end
  end
end

I got the same blasted error mentioned here. I checked my code carefully for mistakes, but I couldn't solve the problem. I found this issue, and as suggested I did:

.../auction_umbrella$ rm -rf _build

Then I did

.../auction_umbrella$ mix phx.server

which recompiled all the files, and problem solved.

[raymondsiu]

I had the same issue following with the Phoenix in Action book. Having the phoenix_ecto package and cleaning up the _build folder solved the issue for me.
In my case I didn't build an umbrella app so my Phoenix layer is separate from my Model and separate from my User/Auth layer.

[shanlalit]

I generate the app with --no-ecto
So I needed {:phoenix_ecto, "~> 4.0"} dependency

[mikebobadilla]

removing _build and compiling again worked for me.