How to create a nested rails form?

Question

Ruby on Rails

In general, I forgot. It worked before.
Two models A and B. B is nested in A.

app/models/a.rb

class A < ActiveRecord::Base
  has_one :b, dependent: :destroy
  accepts_nested_attributes_for :b
end

app/models/b.rb

class B < ActiveRecord::Base
  belongs_to :a
end

db/migrate/..rb

class Ab < ActiveRecord::Migration
  def change
    create_table(:as) do |t|
      t.string :a_field, default: ""
    end

    create_table(:bs) do |t|
      t.integer :a_id
      t.string :b_field, default: ""
    end
  end
end

app/controllers/test2_controller.rb

class Test2Controller < ApplicationController
  def edit
    @a = A.all.first
    @b = @a.b
    render "ab_form"
  end

  def update
    # params[:a][:b_attributes] = {}
    # params[:a][:b_attributes][:b_field] = 'a'

    @a = A.where(:id => 1).first
    @a.update(params.require(:a).permit(:a_field, b_attributes: [:b_field]))
    redirect_to edit_a_path
  end

end

views/test2/ab_form.html.haml

=simple_form_for @a, :url => :update_a do |f|
  =f.input :a_field
  =f.simple_fields_for @b do |g|
    =g.input :b_field
  =f.button :submit

config/routes.rb

get		'/тест2/форма', :to => 'test2#edit', :as => :edit_a
  patch	'/тест2/записать', :to => 'test2#update', :as => :update_a

Markup:

<form class="simple_form edit_a" id="edit_a_1" action="/%D1%82%D0%B5%D1%81%D1%822/%D0%B7%D0%B0%D0%BF%D0%B8%D1%81%D0%B0%D1%82%D1%8C" accept-charset="UTF-8" method="post"><input name="utf8" type="hidden" value="✓"><input type="hidden" name="_method" value="patch"><input type="hidden" name="authenticity_token" value="Yuh+fMJMZ+OdHClUcyHoFLdBXp/w/Pnio4kAP64wVgL+uDi/GCZtvCO1p5opB44X3lSE8/4LjcBuwZLgxNzNJg==">  <div class="input string optional a_a_field"><label class="string optional" for="a_a_field">A field</label><input class="string optional" type="text" value="asff2" name="a[a_field]" id="a_a_field"></div>
            <div class="input string optional a_b_b_field"><label class="string optional" for="a_b_b_field">B field</label><input class="string optional" type="text" value="a" name="a[b][b_field]" id="a_b_b_field"></div>  
            <input type="submit" name="commit" value="Сохранить A" class="button">
          </form>

The result is this. Model A is updated, B is not. If you uncomment the lines in the controller, then everything becomes normal. Those. as I understand it, the catch is that in the markup, the input field of model B has name a[b][b_field], but it should be a[b_attributes][b_field]. What do you think, what could be wrong here. And how everything worked before, I'll never know))

Answer 1

=simple_form_for @a, :url => :update_a do |f|
  =f.input :a_field
  =f.simple_fields_for :b, @b do |g|
    =g.input :b_field
  =f.button :submit