Polymorphic Association is not updating but creating new entry

Question

we just figured out how polymorphic relations are working in Rails 4 but we face a little problem as the relations always creates a new entry instead of updating the existing relation.

Why updating a polymorphic relation creates a new entry instead of updating the existing one?

f.e. with a picture class...

The Picture Model

class Picture < ActiveRecord::Base
  belongs_to :picable, polymorphic: true
end

The Shop Model

class Shop < ActiveRecord::Base
  has_one :picture, :as => :picable
  accepts_nested_attributes_for :picture
end

The Form

  <%= f.fields_for :picture_attributes, @admin_shop.picture||Picture.new do |ff| %>
      <div class="field">
        <%= ff.label :url %><br />
        <%= ff.text_field :url %>
      </div>
  <% end %>

The Controller

  def new
    @admin_shop = Shop.new
  end

  # GET /admin/shops/1/edit
  def edit
  end

  # POST /admin/shops
  # POST /admin/shops.json
  def create
    @admin_shop = Shop.new(admin_shop_params)

    respond_to do |format|
      if @admin_shop.save
        format.html { redirect_to admin_shops_url, notice: 'Shop was successfully created.' }
      else
        format.html { render action: 'new' }          
      end
    end
  end

  def update
    respond_to do |format|
      if @admin_shop.update(admin_shop_params)
        format.html { redirect_to admin_shops_url, notice: 'Shop was successfully updated.' }
      else
        format.html { render action: 'edit' }
      end
    end
  end

Answer 1

add :id to the :picture_attributes section of your params method in the controller. That will almost definitely work. Another method would be to add to your accepts_nested_attributes_for:

accepts_nested_attributes_for :picture, :update_only => true