carrierwave一键多图像上传

我的问题始于 here . 我使用的是Sqlite3,我的模型关系如下所示。

class ImagePost < ActiveRecord::Base
    has_many :attachments
end

我的附件

class Attachment < ActiveRecord::Base
    belongs_to :image_post
    mount_uploader :img, S3uploaderUploader
end

问题是我想让我的用户用一个按钮上传他们的图片。现在,我做到了

<%= form_tag img_upload_create_path, method: "POST", html: { multipart: true } do %>
        <%= hidden_field_tag("image_post_id", @image_post.id ) %>
        <%= hidden_field_tag("user_id", current_user.id ) %>

        <%= file_field_tag 'user_pic', multiple: true, accept: 'image/png,image/gif,image/jpeg' %>

        <%= submit_tag "image-upload", :class => "btn btn-primary btn-lg" %>
      <% end %>

我的image_upload控制器,

class ImgUploadController < ApplicationController
  def create
    @user_img = Attachment.create(
        hasuk_house_id: params[:image_post_id],
        user_id: current_user.id,
        img: params[:user_pic]
        )
  end
end

但当我提交文件时, @user_img.img = nil 我该怎么办?