Encoding with Poison.

controller.ex

defmodule MyApp.MyModelController do                                                                                                                                                            
  use MyApp.Web, :controller                                                                               
  plug :action                                                                                                
  
  def show(conn, params) do
    json conn, Repo.get!(MyModel, params[:id])
  end
end

encoder.ex

defimpl Poison.Encoder, for: MyModel do
  # This is a sigil that produces a list of atoms
  @attributes ~W(id body title user comments inserted_at)
 
  def encode(comment, _options) do
    comment
    |> Map.take(@attributes)
    |> Map.update!(:title, &String.capitalize)
    |> Poison.encode!
  end
end