Ecto custom types: How do I represent a field in a model as a specific type?

Question

I have a schema, where I want one of the fields to be represented in the form of %Cm{value: 1.0} (for centimetre units).

I have defined this custome type:

defmodule Db.Types.Cm do
  alias Units.Cm

  @behavior Ecto.Type
  def type, do: :float

  def cast(%Cm{value: integer}) when is_integer(integer) do
    Cm.new(integer / 1.0)
  end

  def cast(val = %Cm{value: float}) when is_float(float) do
    val
  end

  def cast(number) when is_float(number), do: Cm.new(number)
  def cast(number) when is_integer(number), do: Cm.new(number / 1.0)
  def cast(_), do: :error

  def load(float) when is_float(float), do: Cm.new(float)

  def dump(%Cm{value: float}) when is_float(float), do: float
  def dump(%Cm{value: integer}) when is_integer(integer), do: (integer / 1.0)
  def dump(_), do: :error
end

Following these guidelines from the docs (https://hexdocs.pm/ecto/Ecto.Type.html):

• type should output the name of the DB type
• cast should receive any type and output your custom Ecto type
• load should receive the DB type and output your custom Ecto type
• dump should receive your custom Ecto type and output the DB type

And the following schema:

defmodule Db.Block do
  schema "blocks" do
    field :max_depth, Types.Cm
    timestamps()
  end

  @fields ~w(max_depth)a

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, @fields)
  end
end

Now I try to save blocks to the db:

defmodule Db.BlockHelpers do
  def new_block(attributes \\ %{}) do
    block = Dict.merge(%{
      max_depth: 2
    }, attributes)

    %Block{}
    |> Block.changeset(block)
    |> Repo.insert!
  end
end

iex> new_block()
...> new_block(%{max_depth: Units.Cm.new(5.0)})

However I keep getting errors:

 ** (CaseClauseError) no case clause matching: %Units.Cm{value: 2.0}

---

I've tried various combinations of approaches, but can't seem to get it right. So I'm not 100% sure I understand the documentation.

At the end of the day, I want to be able to pass around a model struct of the form %Block{max_depth: %Units.Cm{value: 1.0}}, where the cm value is stored as a floating point in the database (postgres).

Answer 1

Correct answer provided by Dogbert: was returning value as opposed to {:ok, value}.