P85 Your Turn - (Ecto.CastError) expected params to be a :map, got: `3.0`

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Generators: Contexts and Schemas - p85

Here is the code I have:

catalog.ex:

  def markdown_product(%Product{} = product, unit_price) do
    product
    |> Product.changeset_price(unit_price)
    |> Repo.update()
  end

product.ex:

  def changeset_price(product, unit_price) do
    product
    |> cast(unit_price, [:unit_price])
    |> validate_number(:unit_price, greater_than: 0.0)
  end

My commands:

# mix priv/generators_cli_commands.exs
# or `iex -S mix`, then enter line-by-line

alias Pento.Catalog

random_sku = Enum.random(0 .. 1_000_000)
product_attrs = %{
    name: "Chess",
    description: "The classic strategy game",
    unit_price: 10.0,
    sku: random_sku,
}

{:ok, product} = Catalog.create_product(product_attrs)

unit_price = 3.0

Catalog.markdown_product(product, unit_price)

It runs OK until the last line, which gives:

** (Ecto.CastError) expected params to be a :map, got: `3.0`

There is some generated @spec, but it doesn’t mean much to me right now:

  @spec markdown_product(
          %Pento.Catalog.Product{optional(atom) => any},
          :invalid | %{optional(:__struct__) => none, optional(atom | binary) => any}
        ) :: any

A few questions:

  • How can I tell that it expects a :map for unit_price?
    • Why does it expect a map?
  • How can I tell Elixir that I expect this second paramter to be a float?

Thanks!