= simple_form_for(@book) do |f|
  = f.error_notification
  = f.error_notification message: f.object.errors[:base].to_sentence if f.object.errors[:base].present?
  .form-inputs
    = f.input :title, as: :string, label: 'Book Title'
    = f.input :release_year, as: :string, label: 'Release Year'
    %br
    = f.simple_fields_for :book_authors do |f2|
      = render 'book_author_fields', f: f2
    .links
      = link_to_add_association 'Add Author', f, :book_authors
  .form-actions
    = f.button :submit