how to use carrierwave

_form.html.erb

  <%= form_for Attachment.new do |f| %>
    <%= f.label :file, '上传文档', class: 'btn btn-primary btn-sm' %>
    <%= f.file_field :file %>
    <span class="text-danger">文件大小不能超过20M</span>
    <%= f.submit 'save', class: 'btn btn-primary btn-sm' %>
  <% end %>

attachment.rb

class Attachment < ActiveRecord::Base
  UPLOAD_LIMIT = 20.megabytes
  mount_uploader :file, FileUploader , mount_on: :name

  belongs_to :attachmentable, polymorphic: true

  validates :file, presence: true
  validate :file_size_validation, if: 'file?'

  before_save do
    self.size = self.file.size
    self.content_type = self.file.content_type
  end


  def size_display
    kb = size.to_f / 1.kilobyte
    if kb / 1.kilobyte > 1
      mb = kb / 1.kilobyte
      "#{ '%.2f' % mb }MB"
    else
      "#{ '%.2f' % kb }KB"
    end
  end

  private

  def file_size_validation
    if file.size > UPLOAD_LIMIT
      errors.add(:file, "不能超过#{UPLOAD_LIMIT / 1.megabytes}MB")
    end
  end
end

base_uploader.rb

class BaseUploader < CarrierWave::Uploader::Base
  storage :file

  def sanitize_filename(filename)
    filename.strip.tap do |name|
      # NOTE: File.basename doesn't work right with Windows paths on Unix
      # get only the filename, not the whole path
      name.sub! /\A.*(\\|\/)/, ''
      # Finally, replace all non alphanumeric, underscore
      # or periods with underscore
      name.gsub! /[^\w\.\-]/, '_'
    end
  end

  def store_dir
    # uploads/file/000/000/001/
    "uploads/#{mounted_as}/#{('%09d' % model.id).scan(/\d{3}/).join('/')}"
  end
end

file_uploader.rb

class FileUploader < BaseUploader
end

migration.rb

class CreateAttachments < ActiveRecord::Migration
  def change
    create_table :attachments do |t|
      t.integer :attachmentable_id
      t.string :attachmentable_type
      t.string :name
      t.string :content_type
      t.integer :size
      t.integer :download_count, default: 0
      t.datetime :deleted_at

      t.timestamps null: false
    end
  end
end