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)
    timestamps()
  end

  ...
end

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