The “Give It a Try” exercise “Add a Custom Validation” challenges the reader to:

First, add a custom validation to the Product schema’s changeset that validates that :unit_price is greater than 0.00.

This step is unnecessary because the unit price validation was included in the code on page 77.

def changeset(product, attrs) do
  product
  |> cast(attrs, [:name, :description, :unit_price, :sku])
  |> validate_required([:name, :description, :unit_price, :sku])
  |> unique_constraint(:sku)
  |> validate_number(:unit_price, greater_than: 0.0)
end

The rest of page 77 shows what happens when a changeset includes an invalid unit price.