Mapping/transforming user input from a form that does not directly map to a schema?


I’m new to Phoenix/Ecto and was wondering how to map/transform user input that does not directly map to a schema.

Let’s say I have a schema that represents a Product that is persisted in the database. In that schema is a field that contains the price for that product stored as an integer of the smallest unit of currency, say cents.

When adding a new product, or updating an existing one, it would be better if the user could enter the price in two separate fields as dollars and cents. Instead of total cents.

What is the best/proper way of handling the transformation from dollars and cents to total cents that can be assigned to a Product before inserting it into the database? And then the total cents → dollars + cents when updating an existing Product?

I’ve been reading about creating an embedded schema for this purpose. Or adding virtual fields to the Product schema. Maybe adding a custom Ecto.Type + creating Phoenix form input helpers?

How are these kinds of problems usually solved?


1 Like

Hi :wave:

Ecto Type looks like a good fit for this case. The advantage of it is that is pluggable so you use it on similar fields if you need.

defmodule PriceType do
  use Ecto.Type

  def type, do: :integer

  def cast(%{unit: unit, cents: cents}) do
    {:ok, to_price(unit, cents)}

  # This handle params that comes directly from Phoenix controllers or LiveView.
  def cast(%{"unit" => unit, "cents" => cents}) do
    {:ok, to_price(unit, cents)}

  def cast(price) when is_integer(price) do
    {:ok, price}

  def cast(_), do: :error

  # You might need to handle more cases when unit and cents are not binary.
  defp to_price(unit, cents) when is_binary(unit) and is_binary(cents) do
    String.to_integer(unit <> cents)

  defp to_price(unit, cents) do
    raise "#{unit} and #{cents} are not Strings"

  def load(price) when is_integer(price), do: {:ok, price}

  def dump(price), do: {:ok, price}

Other options would be doing it on the changeset level or even on the controller/Live view layer. It really depends on how you want/prefer this data to passed around in your application.

People also like to use Decimal — Decimal v2.0.0 to handle it and there is also this library that you can take some inspiration GitHub - elixirmoney/money: Elixir library for working with Money safer, easier, and fun... Is an interpretation of the Fowler's Money pattern in fun.prog..

It also common to use Money masks on User inputs so that the value comes to your back-end as: $1000,20. This is doable in the same Ecto Type above just adding a new clause to cast/1