Hao's Thoughts

Ruby, Rails, Objective-C and everything else

Rails 4 Gotcha: Nested Form Created New Child Object Every Time Parent Object Is Updated

I’ve been playing with Rails 4 recently, the strong parameter, as you all know, is a new feature just appeared in Rails 4. It will get really tricky when you are posting a nested form.

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# question.rb
class Question < ActiveRecord::Base
  has_one :answer, dependent: :destroy
  accepts_nested_attributes_for :answer, reject_if: Proc.new {|a| a["body"].blank?}
end

# answer.rb
class Answer < ActiveRecord::Base
  belongs_to :question
end



# questions_controller.rb
class QuestionsController < ApplicationController
  def update
      @question = Question.find(params[:id])
      
      if @question.update_attributes(question_params)
        redirect_to questions_path, notice: "Question updated."
      else
        render :edit
      end
  end

  protected
  def question_params
      params.require(:question).permit(:content, answer_attributes: [:body])
  end
end

# questions/edit.html.haml

= form_for @question do |f|
  = f.text_field :content

  = f.fields_for :answer, @question.answer || @question.build_answer do |builder|
    = builder.text_field :body

  = f.submit "Update", disable_with: "Updating ..."

The above code are really straightforward Rails 4 code for updating a given question with its answer, the problem right now is every time I updated the question, no matter if updated the associated answer or not, it will destroy the old answer object, and create a new answer object for me, that’s really annoying, the gotcha here is you need have the answer’s id field go through for strong parameter, otherwise, it will be filtered out, and the old answer will be destroyed, and a new answer object will be created silently. The revision is something like:

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# questions_controller.rb
class QuestionsController < ApplicationController
  ...

  protected
  def question_params
      params.require(:question).permit(:content, answer_attributes: [:id, :body])
  end
end

Happy coding.

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