I have an Elixir application using Ecto, and I need to dynamically cast and validate embedded schemas based on a type field. My Discount schema has a field discount_params that can be different structures depending on the value of the type field.

I don’t want to change the field type from :map and want to leave it as it is.

Here is the base function:

defmodule Discounts.Discount do

  schema "discounts" do
    field :type, :string
    field :global, :boolean
    field :discount, :integer
    field :discount_params, :map
    field :latest_event, :string, virtual: true, default: ""

    has_many :event_discounts, EventDiscount, on_replace: :delete
    has_many :events, through: [:event_discounts, :event]

    timestamps()
  end

  @type id_t() :: String.t() | non_neg_integer()
  @type t ::
          %__MODULE__{
            id: non_neg_integer(),
            type: String.t(),
            global: boolean(),
            discount: non_neg_integer(),
            discount_params: %{String.t() => String.t() | non_neg_integer()},
            inserted_at: Date.t(),
            event_discounts: [EventDiscount.t()],
            events: [Event.t()]
          }
          | %__MODULE__{}

  @required_params ~w(type discount discount_params)a
  @optional_params ~w(global)a

  defp changeset(discount, params) do
    discount
    |> cast(params, @required_params ++ @optional_params)
    |> cast_assoc(:event_discounts)
    |> validate_required(@required_params)
    |> validate_inclusion(:type, ["Package", "Volume"])
    |> cast_discount_params(params)
  end

  defp cast_discount_params(changeset, params) do
    type = get_field(changeset, :type)

    schema_module = Module.concat(Discounts.DiscountType, DiscountParams.schema(type))
     # return example Discounts.DiscountType.Volume.Params

    if schema_module do
      embed_params = Map.get(params, "discount_params", %{})
      embed_changeset = schema_module.changeset(%schema_module{}, embed_params)

      if embed_changeset.valid? do
        params = apply_changes(embed_changeset)
        change(changeset, discount_params: params)
      else
        Enum.reduce(embed_changeset.errors, changeset, fn {field, error}, acc ->
          add_error(acc, field, error)
        end)
      end
    else
      changeset
    end
  end

And I have two discount types:

defmodule SG.Shop.Discounts.DiscountType.Package do
  use SG.Shop.Discounts.DiscountType

  alias SG.Shop.Discounts.Discount
  alias SG.Shop.Events.ProductType

  @name DiscountType.name(__MODULE__)

  defmodule Params do
    use DiscountParams

    @primary_key false
    embedded_schema do
      field :product_type_a, :string
      field :product_type_b, :string
    end

    @required_params ~w(product_type_a product_type_b)a
    @optional_params ~w()a

    def changeset(discount_params, params \ %{}) do
      discount_params
      |> cast(params, @required_params ++ @optional_params)
      |> validate_required(@required_params)
      |> validate_inclusion(:product_type_a, ProductType.all_types())
      |> validate_inclusion(:product_type_b, ProductType.all_types())
    end
  end
end

   
    
defmodule SG.Shop.Discounts.DiscountType.Package do
  use SG.Shop.Discounts.DiscountType

  alias SG.Shop.Discounts.Discount
  alias SG.Shop.Events.ProductType

  @name DiscountType.name(__MODULE__)

  defmodule Params do
    use DiscountParams

    @primary_key false
    embedded_schema do
      field :product_type_a, :string
      field :product_type_b, :string
    end

    @required_params ~w(product_type_a product_type_b)a
    @optional_params ~w()a

    def changeset(discount_params, params \ %{}) do
      discount_params
      |> cast(params, @required_params ++ @optional_params)
      |> validate_required(@required_params)
      |> validate_inclusion(:product_type_a, ProductType.all_types())
      |> validate_inclusion(:product_type_b, ProductType.all_types())
    end
  end
end

I have tried the above approach, but I am facing issues with dynamically setting the schema_module. How can I achieve this so the validation goes through the embedded schema based on the type we get? And discounts discount_type is created only with this fields not with all given from params

    defmodule Discounts.DiscountType.Volume do
      
      defmodule Params do
        use DiscountParams
    
        @primary_key false
        embedded_schema do
          field(:n, :integer)
          field(:product_type, :string)
        end
    
        @required_params ~w(n product_type)a
        @optional_params ~w()a
    
        def changeset(discount_params, params \ %{}) do
          discount_params
          |> cast(params, @required_params ++ @optional_params)
          |> validate_required(@required_params)
          |> validate_inclusion(:product_type, ProductType.all_types())
          |> validate_number(:n, greater_than: 0, less_than_or_equal_to: 100)
        end
      end
    end