Thanks for the different approaches - very useful!

One more question though…

What if :allowed_types is itself an attribute of the type? In other words, I can set the allowable types on the entity with a call like this:

# .create(:name, :allowed_types)
ActivityStereotype.create("Meetings", [:one_on_one, :team, :board_room]) 

And now I want to make sure that if the :default_type is also set (on the same entity) it must be one of the types allowed when I created the entity. See it’s self-referential. First you set the allowed types, then you make sure the default type is one of the allowed types.

# .create(:name, :allowed_types, :default_type)
ActivityStereotype.create("Meetings", [:one_on_one, :team, :board_room], :team) 

I’m having some trouble getting the bit that says “given a new :default_type, check to make sure it’s one of the :allowed_types that I just provided”.

My solution is this custom validator, but it seems a bit hardcoded.

defmodule COE.Walk.ActivityStereotype.Validations.TypesAreAllowed do
  @moduledoc """
  Custom validator that checks to make sure a list of types includes one or more values from @allowed_types.
  """

  def init(opts) do
    if is_atom(opts[:attribute]), do: {:ok, opts}, else: {:error, "attribute must be an atom!"}
  end

  def validate(changeset, opts) do
    value = Ash.Changeset.get_attribute(changeset, opts[:attribute])
    allowed_types = Ash.Changeset.get_attribute(changeset, :allowed_types)

    #if value != nil && Enum.all?(value, &(&1 in allowed_types)) do
    if value != nil && value in allowed_types do
      :ok
    else
      {:error, field: opts[:attribute], message: "allowed types must be one of #{allowed_types}"}
    end
  end
end

My very first attempt (which did not work) was:

  validations do
    validate attribute_in(:default_type, :allowed_types)
  end

And it just seemed like there ought to be a simple solution to this that I haven’t discovered yet (beyond the custom validator).