Ecto Model Schema Extension

product.ex

defmodule Nectar.ModelExtension do
  defmacro __using__(_opts) do
    quote do
      Module.register_attribute(__MODULE__, :schema_changes, accumulate: true)
      import Nectar.ModelExtension, only: [add_to_schema: 1]
      @before_compile Nectar.ModelExtension
    end
  end
    
  defmacro add_to_schema([do: block]) do
    schema_change = Macro.escape(block)
    quote bind_quoted: [schema_change: schema_change] do
      Module.put_attribute(__MODULE__, :schema_changes, schema_change)
    end
  end
    
  defmacro __before_compile__(_env) do
    quote do
      defmacro extensions do
        @schema_changes
      end
    end
  end
end

defmodule Nectar.ExtendProduct do
  use Nectar.ModelExtension
    
  add_to_schema do: (field :special, :boolean, virtual: true)
end

defmodule Nectar.Product do
  import Nectar.ExtendProduct
  
  schema do
    extensions
  end
  
  ## Add #special boolean to get casted
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, ~w(special))
  end
end