title: Ruby on rails How to remove added columns and insert new columns through a migration file
body: <p>HI I created a Ruby on rails migration file as follows and in the first stage I created tables </p>
  
  <p>then I want to add columns and remove some columns and I modified it as follows </p>
  
  <pre><code>class CreateMt940Batches &lt; ActiveRecord::Migration
    def change
      create_table :mt940_batches do |t|
       t.string :account_number
       t.string :transaction_reference_number
       t.string :information_to_account_owner
       t.string :file_name
       t.binary :raw_data_transaction
       t.string :sha1_checksum
       t.timestamps
     end
  
     def self.down
       remove_column :account_number, :transaction_reference_number, :information_to_account_owner
     end
  
     def self.up
       add_column :mt940_batches, :created_by, :updated_by, :integer
     end 
   end
    end
  </code></pre>
  
  <p>but when I ran rake db:migrate nothing has happens. How to accomplish this task . I want to change the model already created as well from this migration file. Um looking a way to do this. Thank you in advance</p>