Rails 3 Nested Model Form, 2 levels deep using accepts_nested_attributes_for

问题

My nested model form is working great on the first level deep. But I was under the impression that you could go many levels deep using accepts_nested_attributes_for. But when I try the code below, the "Image" attributes are attached to the top level "Question" model and it breaks upon form submission with an unknown attribute "Image" error.

I could do the inserts all by hand using the form data but if Rails can handle it automatically, it would be better for obvious reasons.

What am I doing wrong? I tried changing |af| in the "fields for :image do" to its own unique name but it didn't have any effect.

Models:

class Question < ActiveRecord::Base
  has_one :answer
  accepts_nested_attributes_for :answer
end

class Answer < ActiveRecord::Base
  belongs_to :question
  has_one :image
  accepts_nested_attributes_for :image
end

class Image < ActiveRecord::Base
  belongs_to :answer
end

Controller:

def new
    @question = Question.new
    answer = @question.build_answer
    image = answer.build_image

    @case_id = params[:id]

    render :layout => 'application', :template => '/questions/form' 
end

def create
  question_data = params[:question]
  @question = Question.new(question_data)
  if @question.save
  ...
end

View:

= form_for @question, :html => {:multipart => true} do |f|

  = f.label :text, "Question Text:"
  = f.text_area :text, :rows => 7

  %br
  %br

  =f.fields_for :answer, do |af|
    = af.label :body, "Answer Text:"
    = af.text_area :body, :rows => 7

    %br
    %br

    = f.fields_for :image do |af|
      = af.label :title, "Image Title:"
      = af.text_field :title

      %br

      = af.label :file, "Image File:"
      = af.file_field :file

      %br

      = af.label :caption, "Image Caption:"
      = af.text_area :caption, :rows => 7

  = hidden_field_tag("case_id", value = @case_id)

  = f.submit

回答1:

I think you've got the form variables slightly mixed up. It should be:

= form_for @question, :html => {:multipart => true} do |f|

  = f.label :text, "Question Text:"
  = f.text_area :text, :rows => 7

  %br
  %br

  =f.fields_for :answer, do |af|
    = af.label :body, "Answer Text:"
    = af.text_area :body, :rows => 7

    %br
    %br

    = af.fields_for :image do |img_form|
      = img_form.label :title, "Image Title:"
      = img_form.text_field :title

      %br

      = img_form.label :file, "Image File:"
      = img_form.file_field :file

      %br

      = img_form.label :caption, "Image Caption:"
      = img_form.text_area :caption, :rows => 7

  = hidden_field_tag("case_id", value = @case_id)

  = f.submit

Notice how form_for ... do |f| spawns f.fields_for ... do |af|, which in turns spawns af.fields_for ... do |img_form|.

The key is the second fields_for. It should be af.fields_for :image do |img_form| rather than f.fields_for :image do |img_form|.

来源：https://stackoverflow.com/questions/5932934/rails-3-nested-model-form-2-levels-deep-using-accepts-nested-attributes-for