Attributes on `many_to_many` join/through resources.

defmodule Formula do
  use Ash.Resource,
    data_layer: Ash.DataLayer.Ets
  
  attributes do
    uuid_primary_key :id

    create_timestamp :created_at
    update_timestamp :updated_at
  end

  actions do
    defaults([:create, :read, :update, :destroy])

    create :define do

      argument :formula, {:array, :map}

      change manage_relationship(:formula, :reactants,
               on_lookup: :relate,
               on_no_match: :create,
               on_match: :update,
               on_missing: :unrelate
             )
    end

  end

  relationships do
    many_to_many :reactants, Reactant do
      through FormulaReactant
      source_attribute_on_join_resource :formula_id
      destination_attribute_on_join_resource :reactant_id
    end
  end

end
defmodule FormulaReactant do
  use Ash.Resource,
    data_layer: Ash.DataLayer.Ets
  attributes do
    attribute :quantity, :decimal do
      allow_nil? false
      constraints [min: 0.0]
    end

    attribute :unit, :string do
      allow_nil? false
      constraints trim?: true, allow_empty?: false, min_length: 1
    end
  end

  actions do
    defaults [:create, :read, :update, :destroy]

  end

  relationships do
    belongs_to :formula, Formula, primary_key?: true, allow_nil?: false
    belongs_to :reactant, Reactant, primary_key?: true, allow_nil?: false
  end

end
defmodule Reactant do
  use Ash.Resource,
    data_layer: Ash.DataLayer.Ets
  
  attributes do
    uuid_primary_key :id

    attribute :name, :string

    create_timestamp :created_at
    update_timestamp :updated_at
  end

  actions do
    defaults([:create, :read, :update, :destroy])
  end
end

Formula
|>Ash.Changeset.for_create(:define, %{formula: [%{name: "pinata"}]})
|>Api.create()