使用嵌套属性的Rails文件上载，文件保存时“file_content”为空

Question

我一直在关注本教程，将文件上传到db Rails File Upload

当文件保存并且我使用file_contents上传文件时，问题是nested_attributes字段为空。

subject.rb // model

class Subject < ApplicationRecord
  has_many  :documents
  accepts_nested_attributes_for :documents
end

document.rb // model

class Document < ApplicationRecord
  belongs_to  :subject

def initialize(params = {})
  @file = params.delete(:file)
  super
  if @file
    self.filename = sanitize_filename(@file.original_filename)
    self.content_type = @file.content_type
    self.file_contents = @file.read
  end
end

private

  def sanitize_filename(filename)
    return File.basename(filename)
  end
end

_form.html.erb //查看

<%=  form_for(subject) do |f| %>
.......
   <div class="uk-form-row">
      <%= f.fields_for :documents do |d| %>
         <%= d.file_field :file, class:"uk-width-1-1 uk-form-large", placeholder:"upload document" %>
      <% end %>
   </div>
............
<% end %>

subjects_controller.rb

def new
  @subject = Subject.new
  @subject.documents.build
end

def create
  @subject = Subject.new(subject_params)

  respond_to do |format|
    if @subject.save
      format.html { redirect_to @subject, notice: 'Lesson was successfully created.' }
    else
      format.html { render :new }
    end
  end
end

private
  def subject_params
    params.require(:subject).permit(:title, documents_attributes:[:file])
  end

我是以正确的方式吗？我错过了什么？我需要你的想法。

谢谢，