如何在Ruby on Rails中单独呈现XML错误消息？

def create
  @post = Post.new(post_params)
  if @post.save
    render json: @post, status: :created
  else
    render_error_message(@post.errors.full_messages)
  end
end

def render_error_message(errors)
  render xml: { error: errors }.to_xml, status: :unprocessable_entity
end

config.action_dispatch.default_headers = { 'Content-Type' => 'application/xml' }

Rails.application.routes.draw do
  resources :posts, defaults: { format: :xml }
end

def self.from_xml(xml)
  post = Post.new
  post.title = xml.at_css('title').text
  post.body = xml.at_css('body').text
  post
end

gem 'nokogiri'