Rails db:migrate cannot mass assign protected attributes

Question

I am learning Rails with the book Agile Web Development with Rails 4th edition.

Given the code for the migration below:

class CombineItemsInCart < ActiveRecord::Migration
  def up
    Cart.all.each do |cart|
      sums = cart.line_items.group(:product_id).sum(:quantity)

      sums.each do |product_id, quantity|
        if quantity > 1
          cart.line_items.where(product_id: product_id).delete_all
          cart.line_items.create(product_id: product_id, quantity: quantity)
        end
      end
    end
  end

  def down
    LineItem.where("quantity>1").each do |line_item|
      line_item.quantity.times do
        LineItem.create(cart_id: line_item.cart_id, product_id: line_item.product_id, quantity: 1)
      end
      line_item.destroy
    end
  end
end

The following error occurs:

==  CombineItemsInCart: migrating =============================================
rake aborted!
An error has occurred, this and all later migrations canceled:

Can't mass-assign protected attributes: quantity/home/richard/projects/pickaxe/mini-projects/depot-app/db/migrate/20130607003533_combine_items_in_cart.rb:9:in `block (2 levels) in up'
/home/richard/projects/pickaxe/mini-projects/depot-app/db/migrate/20130607003533_combine_items_in_cart.rb:6:in `each'
/home/richard/projects/pickaxe/mini-projects/depot-app/db/migrate/20130607003533_combine_items_in_cart.rb:6:in `block in up'
/home/richard/projects/pickaxe/mini-projects/depot-app/db/migrate/20130607003533_combine_items_in_cart.rb:3:in `each'
/home/richard/projects/pickaxe/mini-projects/depot-app/db/migrate/20130607003533_combine_items_in_cart.rb:3:in `up'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)

Now I read somewhere that this book was written before attr_accessible was required by default, but it hasnt really touched on how to use it properly yet. I have tried adding :line_item or :line_items to my attr_accessible line in the Cart model, but no luck.

Could someone educate me on what is happening here?

Answer 1

Can't mass-assign protected attributes: quantity

try attr_accessible :quantity

you will need to list all of the attributes in that list.

Answer 2

You need to make the attributes accessible. In the model:

class Object ActiveRecord::Base
  attrib_accessible :attrib1, :attrib2, :attrib3
end

Obviously you would replace the attrib1 etc. with your model's attributes.

Answer 3

You try to access mass attributes from your migration. When you want to access database table attributes from your code then you need to allowing mass assignment of that attributes to tell your model that you can able to assign data for the field by the code. For this pusrpose, just add the desired field as a attr_accessible and for your issue specific solution is bellow :

class LineItem < ActiveRecord::Base

attr_accessible :quantity, :product_id, :cart_id

end