ruby-on-rails – Rails：调用to_json中的模型方法(：methods =&

当我查看Json响应时,它只显示image_url而不是thumb_url
请指导我在这里做错了什么.

型号：

class Post < ActiveRecord::Base
  include Rails.application.routes.url_helpers

    validates :image,presence: true

      has_attached_file :image,styles: { :medium => "640x",thumb: "100x100#" } # # means crop the image
        validates_attachment_content_type :image,:content_type => /Aimage/.*Z/

  def image_url
   relative_path =  image.url(:medium)
   self.add_host_prefix relative_path
  end

 def thumb_url
   relative_path = image.url(:thumb)
   self.add_host_prefix relative_path
 end

  def add_host_prefix(url)
    URI.join(root_url,url).to_s
  end
end

控制器：

class Api::ImagesController < ApplicationController

  def index
    @posts =  Post.all.order(id: :desc)
    paginated_records = @posts.paginate(:page => params[:page],:per_page => params[:per_page])
    @posts = with_pagination_info( paginated_records )
    render :json => @posts.to_json(:methods => [:thumb_url],:methods =>[:image_url])
  end
end

这是我的Json回应：

"data": [{
"id": 23,"caption": "World Top View","created_at": "2015-09-17T14:10:57.278Z","updated_at": "2015-09-17T14:10:57.278Z","image_file_name": "world.topo.bathy.200401.3x21600x10800.jpg","image_content_type": "image/jpeg","image_file_size": 29698041,"image_updated_at": "2015-09-17T14:10:36.975Z","image_url": "http://localhost:3000/system/posts/images/000/000/023/medium/world.topo.bathy.200401.3x21600x10800.jpg?1442499036"}