title: Where is migration status saved?
body: <p>My question is very much related to this one <a href="https://stackoverflow.com/questions/25121347/should-i-delete-migration-after-rollback">Should I delete migration after rollback</a>.  </p>
  
  <blockquote>
    <p>I had my original migrate file <em>20140731141350_create_users.rb</em></p>
  
  <pre><code>class CreateUsers &lt; ActiveRecord::Migration
    def change
      create_table :users do |t|
        t.string :email
        t.string :password
  
        t.timestamps
      end
    end
  end
  </code></pre>
    
    <p>To which I needed to add a <em>salt</em> column, so I created the migration
    <em>20140804125449_add_salt_colum_to_users.rb</em></p>
  
  <pre><code>class AddSaltColumToUsers &lt; ActiveRecord::Migration
    def change
      add_column :users, :salt, :string
    end
  end
  </code></pre>
    
    <p>But during development I realised the <em>salt</em> column wasn't necessary
    and performed</p>
  
  <pre><code>rake db:migrate:down VERSION=20140731141350
  </code></pre>
    
    <p>Now I am left with an unused
    <em>20140804125449_add_salt_colum_to_users.rb</em> migrate file.</p>
  </blockquote>
  
  <p>My question is if I don't delete this migration file, where is this "down" status of this migration saved to?  The migration file says add_column so if I run a db:migrate again how will it know that this specific file was migrated down?</p>