Format error messages for champs

dossier.rb

validates_associated :champs, :champs_private
# ->  Rails displays: "Champs is invalid"

validates_associated :champs, :champs_private, message: -> (object, _data) {
  (object.champs + object.champs_private)
    .reject(&:valid?)
    .flat_map do |champ|
      champ.errors.full_messages.map do |message|
        "#{champ.libelle} : #{message}"
      end
    end
    .to_sentence
}
# ->  Rails displays: "Champs Carte d'identité : Le fichier n’est pas d’un type accepté"