Cannot convert struct to map with Jason @derive

I have a class Company (Echo.Schema)

defmodule MyProject.Company do
  use Ecto.Schema

  @derive {Jason.Encoder, only: [:name]}
  @primary_key {:id, :binary_id, autogenerate: true}
  @foreign_key_type :binary_id
  schema "companies" do
    field(:name, :string)
    field(:count, :integer, virtual: true)
    has_many(:car, Car, foreign_key: :company_id, references: :id)


When I try to convert a Company to a Map, it triggers this error:

company |> Map.from_struct()

[error] GenServer #PID<0.820.0> terminating
** (RuntimeError) cannot encode metadata from the :__meta__ field for MyProject.Fleet to JSON. This metadata is used internally by Ecto and should never be exposed externally.

You can either map the schemas to remove the :__meta__ field before encoding to JSON, or explicit list the JSON fields in your schema:

    defmodule MyProject.Fleet do
      # ...

      @derive {Jason.Encoder, only: [:name, :title, ...]}
      schema ... do

I don’t understand the error since @derive is set in the model ? Did I forgot to do something ?

I have set Jason as Phoenix JSON parsing lib in config/config.ex :

# Use Jason for JSON parsing in Phoenix
config :phoenix, :json_library, Jason
1 Like

Map.from_struct() is not related to Jason.Encoder. Json.Encoder is a protocol for converting structs to JSON, not maps.

However, both are working for me in this example:

defmodule MyStruct do
  use Ecto.Schema
  @derive {Jason.Encoder, only: [:name]}

  schema "my_struct" do
    field(:name, :string)
    field(:other, :string)


iex(1)> Jason.encode(%MyStruct{})
{:ok, "{\"name\":null}"}
iex(2)> Map.from_struct(%MyStruct{})
  __meta__: #Ecto.Schema.Metadata<:built, "my_struct">,
  id: nil,
  name: nil,
  other: nil

I redacted too much code and forgot to tell that this was passed to push_event() right after.

  company |> Map.from_struct()

So my guess is that push_event is using Jason to turn the map into JSON. And because I already turned the Company into a Map before, Jason doesn’t clean any fields and __meta__ is still there, triggering this error.

Thanks for pointing out what I was not understanding correctly!