Validation functions for Echo.Changeset.

The Ecto schema changes can be validated to conform to types in t() and to fulfil appropriate preconditions.

defmodule User do
  use Ecto.Schema
  use Domo, ensure_struct_defaults: false

  import Ecto.Changeset
  import Domo.Changeset

  schema "users" do
    field :first_name, :string
    field :last_name, :string
    field :age, :integer
  end

  @type t :: %__MODULE__{
    first_name :: String.t() | nil,
    last_name :: String.t(),
    age :: age()
  }

  @type age :: pos_integer()
  precond age: &validate_age/1

  @max_age 150
  defp validate_age(age) when age < @max_age, do: :ok
  defp validate_age(_age), do: {:error, "age should be in 1..#{@max_age}"}

  def changeset(user, attrs) do
    user
    |> cast(attrs, typed_fields())
    |> validate_required(required_fields())
    |> validate_type(maybe_filter_precond_errors: true)
  end
end

The ensure_struct_defaults: false option disables the validation of defaults to match to t() type at compile time. That is useful because any Ecto schema has all fields set to nil by default.

typed_fields/1 and required_fields/1 are added automatically to the current module by using Domo.

The first_name field is not required to have a value in the changeset because it has nil as one of the possible types defined.

validate_type/2 function automatically adds type ensurance errors to the changeset. The maybe_filter_precond_errors: true option enables the filtering of the precondition error message for :age field. That error is ready to be communicated to the user.