I’m trying to change the values of some fields to camelCase with middleware like this:

defmodule Middlewares.CamelCaseField do
  @behaviour Absinthe.Middleware
  def call(resolution, _) do
    %{resolution | errors: Enum.flat_map(resolution.errors, &handle_error/1)}
  end

  defp handle_error(%{field: field} = error) do
    %{error | field: camel(field)}
  end

  defp handle_error([]), do: []

  defp handle_error(error) when is_list(error) do
    case Keyword.get(error, :field) do
      nil -> error
      field -> Keyword.put(error, :field, camel(field))
    end
  end

  defp handle_error(error), do: [error]

  def camel(f), do: Atom.to_string(f) |> Recase.to_camel()
end
...
defmodule Schema do
  ....

  def middleware(middleware, field, object) do
    middleware ++ [Middlewares.CamelCaseField]
  end

But now when I run my code I get errors like these:

** (Protocol.UndefinedError) protocol String.Chars not implemented for {:code, :unknown_error}. This protocol is implemented for: Postgrex.Copy, Postgrex.Query, Decimal, DateTime, Date, Version, NaiveDateTime, List, Atom, Time, BitString, Integer, URI, Version.Requirement, Float.

If I remove my middleware everything works, so I’m probably doing something wrong with the middleware, but don’t know what exactly.