foo.ex

defmodule MyApp.Foo do
  use MyApp.Web, :model
  
  @derive [Poison.Encoder]
  schema "foos" do
    belongs_to :user, User
    field :name, :string
  end
end

iex

iex(1)> %MyApp.Foo{} |> Poison.encode!
** (Poison.EncodeError) unable to encode value: {nil, "foos"}
    (poison) lib/poison/encoder.ex:346: Poison.Encoder.Any.encode/2
    (poison) lib/poison/encoder.ex:213: anonymous fn/4 in Poison.Encoder.Map.encode/3
    (poison) lib/poison/encoder.ex:214: Poison.Encoder.Map."-encode/3-lists^foldl/2-0-"/3
    (poison) lib/poison/encoder.ex:214: Poison.Encoder.Map.encode/3
    (poison) lib/poison/encoder.ex:213: anonymous fn/4 in Poison.Encoder.Map.encode/3
    (poison) lib/poison/encoder.ex:214: Poison.Encoder.Map."-encode/3-lists^foldl/2-0-"/3
    (poison) lib/poison/encoder.ex:214: Poison.Encoder.Map.encode/3
    (poison) lib/poison.ex:41: Poison.encode!/2