Invalid single-table inheritance type: Rails

Question

I'm getting this error when trying to create a new Product in my Rails application.

Invalid single-table inheritance type: Movie is not a subclass of Product

How can I resolve this?

controller

class ProductsController < ApplicationController
  before_action :set_product, only: [:show, :edit, :update, :destroy]

  # GET /products
  # GET /products.json
  def index
    @products = Product.all
  end

  # GET /products/1
  # GET /products/1.json
  def show
  end

  # GET /products/new
  def new
    @product = Product.new
  end

  # GET /products/1/edit
  def edit
  end

  # POST /products
  # POST /products.json
  def create
    @product = Product.new(product_params)

    respond_to do |format|
      if @product.save
        format.html { redirect_to @product, notice: 'Product was successfully created.' }
        format.json { render action: 'show', status: :created, location: @product }
      else
        format.html { render action: 'new' }
        format.json { render json: @product.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /products/1
  # PATCH/PUT /products/1.json
  def update
    respond_to do |format|
      if @product.update(product_params)
        format.html { redirect_to @product, notice: 'Product was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @product.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /products/1
  # DELETE /products/1.json
  def destroy
    @product.destroy
    respond_to do |format|
      format.html { redirect_to products_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_product
      @product = Product.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def product_params
      params.require(:product).permit(:title, :artist, :type, :release_date, :category, :publisher, :format, :description, :sale_price, :rental_price)
    end
end

migration

class CreateProducts < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.string :title
      t.string :artist
      t.string :type
      t.date :release_date
      t.string :category
      t.string :publisher
      t.string :format
      t.text :description
      t.decimal :sale_price
      t.decimal :rental_price

      t.timestamps
    end
  end
end

Answer 1

You should not use the type keyword as the column name because it is a reserved word for ActiveRecord.

---

But if you really want to use it, for any reason (like if you don't have control on the DB structure), here is what you should do:

First, make sure your Movie model inherits from the (false-)"abstract" model Product:

class Product < ActiveRecord::Base
  TYPES = %w( Movie )
  before_save :set_type
  validates :type, presence: true, :inclusion => { :in => TYPES }

  def set_type
    raiser "You must override this method in each model inheriting from Product!"
  end

  # ...

class Movie < Product

  def set_type # If you don't implement this method, an error will be raised
    self.type = 'Movie'
  end

And then in your ProductsController you can manage (CRUD) all kind of products.

---

To add a new type of product: you just have to define a new Model inheriting from Product, implement it's set_type method and add the type in the product's Constant:

class Book < Product
  def set_type
    self.type = 'Book'
  end
  #...

class Product < ActiveRecord::Base
  TYPES = %w( Movie Book )

Answer 2

Add self.inheritance_column = nil to your model. type is reserved.

Answer 3

If you don't intend to create a model for your value for 'type', then what is likely happening is 'type' is a reserved word in ActiveRecord.

See http://en.wikibooks.org/wiki/Ruby_on_Rails/ActiveRecord/Naming

"type - This is only used when you have single table inheritance and must contain a class name"

Answer 4

Another way is to name the column differently (product_type would be fine here), and add an attribute alias alias_attribute :type, :product_type in your model definition.

That way you can use @product.type safely, because Rails will substitute product_type when you read from or write to the type attribute.

Tested with Rails 4.1.2.