如何使用Rails 4在编辑表单中创建关联的模型？

Question

I'm currently working on a simple app where I have the following models.

Item:

# app/models/item.rb
class Item < ActiveRecord::Base
  belongs_to :category

  accepts_nested_attributes_for :category
end

Category:

# app/models/category.rb
class Category < ActiveRecord::Base
  has_many :items
end

What I'm trying to do is to create/update an item. I have this controller and form setup.

# app/controller/items_controller.rb
class ItemsController < ApplicationController
  # GET #create
  def new
    @item = Item.new
  end

  # POST #create
  def create
    @item = Item.new ItemParams.build(params)

    if @item.save
      redirect_to @item
    else
      render action: 'new'
    end
  end

  # GET #update
  def edit
    @item = Item.find(params[:id])
  end

  # PATCH #update
  def update
    @item = Item.find(params[:id])

    if @item.update(ItemParams.build(params))
      redirect_to @item
    else
      render action: 'edit'
    end
  end

  class ItemParams
    def self.build(params)
      params.require(:item).permit(:name, :category_id, category_attributes: [:id, :name])
    end
  end
end

Form partial:

# app/views/_form.html.haml
= form_for @item do |f|
  = f.text_field :name

  = f.label :category
  = f.collection_select :category_id, Category.all, :id, :name, { include_blank: 'Create new' }

  = f.fields_for :category do |c|
    = c.text_field :name, placeholder: 'New category'

  = f.submit 'Submit'

You'll notice that in the form, I have a select field and a textbox. What I'm trying to do is to create a new category if the user selects the "New category" in select field and enter the name of the new category in the textfield.

If the setup is correct, I should be able to create a new category from the edit form or change the category. However, I'm getting this error when I try to update an existing item.

ActiveRecord::RecordNotFound - Couldn't find Category with ID=1 for Item with ID=1:

Any help is greatly appreciated. Thanks.

Answer 1

You have to load the category on the new action:

def new
 @item = Item.new
 @item.build_category
end

And to make it work with the edit part I recommend you add the category object to the fields_for helper like so:

f.fields_for :category, @item.category do |c|
 ...

Hope this helps!