code

defmodule LibJasonTest do
  use ExUnit.Case

  defmodule Category do
    @type t :: %__MODULE__{
            id: Integer.t(),
            name: String.t()
          }

    @derive Jason.Encoder
    defstruct [:id, :name]
  end

  defmodule Book do
    @type t :: %__MODULE__{
            id: Integer.t(),
            name: String.t(),
            price: Float.t(),
            categories: [Category.t()],
            created_at: DateTime.t()
          }

    @derive Jason.Encoder
    defstruct [
      :id,
      :name,
      :price,
      :categories,
      :created_at
    ]
  end

  test "encode / decode" do
    book = %Book{}

    dbg(Jason.encode!(book))
  end
end

error info

  1) test encode / decode (LibJasonTest)
     test/lib_jason_test.exs:33
     ** (Protocol.UndefinedError) protocol Jason.Encoder not implemented
 for %LibJasonTest.Book{id: nil, name: nil, price: nil, categories: nil,
 created_at: nil} of type LibJasonTest.Book (a struct), Jason.Encoder pr
otocol must always be explicitly implemented.

     If you own the struct, you can derive the implementation specifying
 which fields should be encoded to JSON:

         @derive {Jason.Encoder, only: [....]}
         defstruct ...

     It is also possible to encode all fields, although this should be u
sed carefully to avoid accidentally leaking private information when new
 fields are added:

         @derive Jason.Encoder
         defstruct ...

     Finally, if you don't own the struct you want to encode to JSON, yo
u may use Protocol.derive/3 placed outside of any module:

         Protocol.derive(Jason.Encoder, NameOfTheStruct, only: [...])   
         Protocol.derive(Jason.Encoder, NameOfTheStruct)
     . This protocol is implemented for the following type(s): Any, Atom
, BitString, Date, DateTime, Decimal, Float, Integer, Jason.Fragment, Ja
son.OrderedObject, List, Map, NaiveDateTime, Time
     code: dbg(Jason.encode!(book))
     stacktrace:
       (jason 1.4.1) lib/jason.ex:164: Jason.encode!/2
       test/lib_jason_test.exs:36: (test)


Finished in 0.1 seconds (0.00s async, 0.1s sync)
1 test, 1 failure