gistfile1.txt

controller code:
  
  def toggle_star(conn, %{"id" => id}) do
    bookmark = Repo.one(Bookmark.by_id_and_user_id(id, user_id(conn)))
    if bookmark do
      Bookmark.changeset(bookmark)
      |> put_change(:starred, !bookmark.starred)
      |> Repo.update!
      render(conn, "toggle_star.json", starred: !bookmark.starred)
    end
  end
  
model:
defmodule Bookmarks.Bookmark do
  use Bookmarks.Web, :model

  schema "bookmarks" do
    field :href, :string
    field :title, :string
    field :description, :string
    field :starred, :boolean
    field :read_later, :boolean
    belongs_to :user, User

    timestamps
  end

  @required_fields ~w(href user_id)
  @optional_fields ~w(title description starred read_later)

  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> validate_length(:title, max: 255)
    |> validate_length(:description, max: 2000)
    |> validate_format(:href, ~r/(^|\s)((https?:\/\/)?[\w-]+(\.[\w-]+)+\.?(:\d+)?(\/\S*)?)/)
    |> validate_length(:href, max: 2000)
  end

  def by_id_and_user_id(id, user_id) do
    from b in Bookmarks.Bookmark,
    where: b.id == ^id and b.user_id == ^user_id,
    select: b
  end
end

error:
[error] #PID<0.582.0> running Bookmarks.Endpoint terminated
Server: localhost:4000 (http)
Request: POST /bookmarks/7/toggle_star
** (exit) an exception was raised:
    ** (Ecto.InvalidChangesetError) could not perform update because changeset is invalid.

* Changeset changes

%{starred: true}

* Changeset params

nil

* Changeset errors

[]

        (ecto) lib/ecto/repo/model.ex:28: Ecto.Repo.Model.update!/4
        (bookmarks) web/controllers/bookmark_controller.ex:48: Bookmarks.BookmarkController.toggle_star/2
        (bookmarks) web/controllers/bookmark_controller.ex:1: Bookmarks.BookmarkController.phoenix_controller_pipeline/2
        (bookmarks) lib/phoenix/router.ex:265: Bookmarks.Router.dispatch/2
        (bookmarks) web/router.ex:1: Bookmarks.Router.do_call/2
        (bookmarks) lib/bookmarks/endpoint.ex:1: Bookmarks.Endpoint.phoenix_pipeline/1
        (bookmarks) lib/plug/debugger.ex:90: Bookmarks.Endpoint."call (overridable 3)"/2
        (bookmarks) lib/phoenix/endpoint/render_errors.ex:34: Bookmarks.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