defmodule EctoDiscriminator.DiscriminatorType do
  @moduledoc """
  A custom type used by the `EctoDiscriminator.Schema` to know which field should be used for its logic.

  ## Schema

  `EctoDiscriminator.DiscriminatorType` field holds name of the diverged module. Field name can be anything.
  User has to mark exactly one field in the base schema with this type. Diverged schemas will inherit this setting.
  Forgetting to add this field will result in compilation error.
  Basic setup:

      field :type, EctoDiscriminator.DiscriminatorType

  You can also use it as a primary key:

      @primary_key {:id, EctoDiscriminator.DiscriminatorType, []}

  You can also make a virtual discriminator:

      field :type, EctoDiscriminator.DiscriminatorType, virtual: true

  > #### Please note {: .info}
  > This will keep most of the functionality intact, however it will disable `where` when fetching data.
  > This means that you can for example still preload this entity inside other schemas,
  > but issuing `Repo.all(Schema.Diverged)` will return all items from `Schema`.

  You can also configure any other options that are supported by `Ecto.Schema`
  (except `default` for obvious reasons).

      field :discriminator, EctoDiscriminator.DiscriminatorType, load_in_query: false

  ## Migrations

  When it comes to the migrations, the underlying type is just `:string`.
  That said, it's completely fine to create migration with string field:

      add :type, :string

  It's possible to be more restrictive and provide an enum inside DB.
  Keep in mind that module name will be stored in form `Elixir.Module.Name`.

  use Ecto.Type

  def type, do: :string
  def cast(value), do: {:ok, value}
  def load(value), do: {:ok, String.to_existing_atom(value)}
  def dump(value) when is_atom(value), do: {:ok, Atom.to_string(value)}
  def dump(_), do: :error