姓と名を別々に入力させて、保存する前に結合する

  <%= f.label :firstName, "姓" %>
  <%= f.text_field :firstName, autofocus: true, required: true, class: 'form-control' %>
  <%= f.label :lastName, "名" %>
  <%= f.text_field :lastName, autofocus: true, required: true, class: 'form-control' %>

class ApplicationController < ActionController::Base
  before_action :configure_permitted_parameters, if: :devise_controller?

  protected
  # strong parameterで姓と名の属性(firstNameとlastName)をpermitする
  def configure_permitted_parameters
    devise_parameter_sanitizer.permit(:sign_up, keys: [:firstName, :lastName])
  end
end

  # 姓と名をDBに保存する前に結合
  before_create :create_name
  def create_name
    self.name = "#{firstName} #{lastName}"
  end