title: Active Admin rollback: "Validation failed: Email has already been taken"
body: <p>I'm at the start of a project in Rails 3.1.3 with a postgresql backend.</p>
  
  <p>I generated a few models and then added the <em>Active Admin</em> gem.
  A bit later I decided that I should combine 2 models, and wanted to rollback to the beginning so I could delete one model and restart the migration with the correct schema.</p>
  
  <p>I typed:</p>
  
  <pre><code>rake db:rollback step=10
  </code></pre>
  
  <p>It went well up to the <em>Active Admin</em> bit, when it aborted the rollback:</p>
  
  <pre><code>$ rake db:rollback
  ==  DeviseCreateAdminUsers: reverting =========================================
  rake aborted!
  An error has occurred, this and all later migrations canceled:
  
  Validation failed: Email has already been taken
  
  Tasks: TOP =&gt; db:rollback
  </code></pre>
  
  <p>I have not added any users, so this is probably the 'admin@example.com' account.</p>
  
  <p><strong>Gemfile</strong></p>
  
  <pre><code>source 'http://rubygems.org'
  
  gem 'rails', '3.1.3'
  
  gem 'pg'
  gem 'json'
  gem 'activeadmin'
  gem 'sass-rails',   '~&gt; 3.1.5'
  gem 'meta_search',    '&gt;= 1.1.0.pre'
  gem "paperclip", :git =&gt; "git://github.com/thoughtbot/paperclip.git"
  gem 'mechanize'
  
  # Gems used only for assets and not required
  # in production environments by default.
  group :assets do
    gem 'less-rails-bootstrap'
    gem 'coffee-rails', '~&gt; 3.1.1'
    gem 'uglifier', '&gt;= 1.0.3'
  end
  
  gem 'jquery-rails'
  </code></pre>