title: Rails 4 migration not adding fields to an existing database
body: <p>I have an order database that I am trying to add two new fields to it</p>
  
  <p>1) seller_id 
  2) buyer_id </p>
  
  <p>when i run the command </p>
  
  <p><strong>rails generate migration AddFieldstoOrders buyer_id:integer seller_id:integer</strong></p>
  
  <p>it shows </p>
  
  <pre><code> invoke  active_record
        create    db/migrate/20140414094632_add_fieldsto_orders.rb 
  </code></pre>
  
  <p>after running rake dd:migrate and rails s it shows</p>
  
  <p>== 20140414094632 AddFieldstoOrders: migrating ================================
  == 20140414094632 AddFieldstoOrders: migrated (0.0000s) =======================</p>
  
  <p>it should give me the seller_id table and buyer_id table. I have double checked the orders database model via the sqlite database model viewer and both fields have not appeared</p>
  
  <p>both fields have been added to my user.rb and order.rb models</p>
  
  <p><strong>user.rb</strong></p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    # Include default devise modules. Others available are:
    # :confirmable, :lockable, :timeoutable and :omniauthable
    devise :database_authenticatable, :registerable,
           :recoverable, :rememberable, :trackable, :validatable
  
     validates :name, presence: true
  
     has_many :listings, dependent: :destroy
     has_many :sales, class_name: "Order", foreign_key: "seller_id"
     has_many :purchases, class_name: "Order", foreign_key: "buyer_id"
  end
  </code></pre>
  
  <p>order.rb</p>
  
  <pre><code>class Order &lt; ActiveRecord::Base
      validates :address, :city, :state, presence: true
  
      belongs_to :listing
      belongs_to :buyer, class_name: "User"
      belongs_to :seller, class_name: "User"
  end
  </code></pre>
  
  <p>I have successfully added a listings id to the order database using 
  rails generate migration AddListingIdToOrders listing_id:integer but now i am unable to add the seller_id and buyer_id fields</p>
  
  <p>migration file</p>
  
  <pre><code>class AddFieldstoOrders &lt; ActiveRecord::Migration
    def change
    end
  end
  </code></pre>