OrderItem quantity error

error.ex

[error] #PID<0.620.0> running Trophus.Endpoint terminated
Server: 127.0.0.1:4000 (http)
Request: POST /add_to_cart
** (exit) an exception was raised:
    ** (ArgumentError) unknown field `quantity` (note only fields are supported in cast, associations are not)
        (ecto) lib/ecto/changeset.ex:248: Ecto.Changeset.type!/2
        (ecto) lib/ecto/changeset.ex:231: Ecto.Changeset.process_param/6
        (elixir) lib/enum.ex:1100: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
        (ecto) lib/ecto/changeset.ex:216: Ecto.Changeset.cast/4
        (trophus) web/controllers/cart_controller.ex:56: Trophus.CartController.add_to_cart/2
        (trophus) web/controllers/cart_controller.ex:1: Trophus.CartController.phoenix_controller_pipeline/2
        (trophus) lib/phoenix/router.ex:281: Trophus.Router.dispatch/2
        (trophus) web/router.ex:1: Trophus.Router.do_call/2

order_item.ex

defmodule Trophus.OrderItem do
  use Trophus.Web, :model

  schema "order_items" do
    belongs_to :dish, Trophus.Dish, foreign_key: :dish_id
    belongs_to :order, Trophus.Order, foreign_key: :order_id
    field :quantity, :integer
    field :total_price, :integer
    field :unit_price, :integer

    timestamps
  end

  @required_fields ~w()
  @optional_fields ~w(quantity total_price unit_price)

  @doc """
  Creates a changeset based on the `model` and `params`.

  If no params are provided, an invalid changeset is returned
  with no validation performed.
  """
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
  end
end