title: Rails 3: Unknown validator: 'PresenceValidator' (ArgumentError)
body: <p>Rails 3.2.19, Ruby 1.9.3p547.</p>
  
  <p>After adding a few gems to development (<code>pry</code> and its various dependencies), any validation in the form <code>validates :some_field, some_default_rails_validator: true</code> fails. Prior to bundling, all of these validations worked perfectly. Rolling back the Gemfile and re-bundling has no effect.</p>
  
  <p>Other folks seem to have had issues with custom (or misspelled) validators, but the failures are all for Rails default validators. For example, running <code>rails c</code> gives me:</p>
  
  <blockquote>
    <p><code>Users/MY_USER/.rvm/gems/ruby-1.9.3-p547@MY_APP/gems/activemodel-3.2.19/lib/active_model/validations/validates.rb:96 in 'rescue in block in validates': Unknown validator: 'PresenceValidator' (Argument Error)</code></p>
  </blockquote>
  
  <p>which traces down to</p>
  
  <blockquote>
    <p><code>from /Users/MY_USER/MY_APP/app/models/document.rb:4 in '&lt;class:SomeModel&gt;'</code></p>
  </blockquote>
  
  <p>In the model file, I have:</p>
  
  <pre><code>validates :title, presence: true
  </code></pre>
  
  <p><code>PresenceValidator</code> is a default Rails validator, and somehow a gem seems to have borked that. I'm not really sure what to do (besides nuking the app from space and reinitializing my development environment).</p>