encoder.ex

defmodule MyApp.Encoder do
  @moduledoc """
    General implementation for the Jason encoder to be used in Ecto schemas
  """

  defmacro __using__(opts) do
    drop_fields = Keyword.get(opts, :drop, [])

    quote do
      defimpl Jason.Encoder, for: [__MODULE__] do
        defp cardinality_to_empty(:one), do: %{}
        defp cardinality_to_empty(:many), do: []

        def encode(struct, opts) do
          struct
          |> Map.from_struct()
          |> Enum.reduce(%{}, fn
            {k, %Ecto.Association.NotLoaded{} = v}, acc ->
              Map.put(acc, k, cardinality_to_empty(v.__cardinality__))

            {k, v}, acc ->
              Map.put(acc, k, v)
          end)
          |> Map.drop([:__meta__, :__struct__] ++ unquote(drop_fields))
          |> Jason.Encode.map(opts)
        end
      end
    end
  end
end