Why do I get an Unknown Primary Key exception for a join table in Rails 4?

Question

These are my models:

class Product
  has_many :line_items
  has_many :orders, :through => :line_items
end

class LineItem 
  belongs_to :order
  belongs_t         


        

Answer 1

The accepted answer got rid of the error message, but I was still unable to save @order.line_items without getting an error telling me [:order_id, :product_id] does not exist.

I finally solved this by deleting the line_items table and recreating it with this migration:

  def change
    create_table :line_items do |t|
      t.references :order
      t.references :product
      t.integer :quantity
      t.timestamps
    end
  end

I hadn't used "references" when I created the table originally, which Rails 3 didn't mind, but made Rails 4 complain.