title: Enabling cache_classes causes undefined method `increment_counter' for ActiveRecord::Relation:Class
body: <p>After upgrade to Rails 3.0.10 I'm getting strange error if I set config.cache_classes=true</p>
  
  <p><code>
  NoMethodError (undefined method <code>increment_counter' for ActiveRecord::Relation:Class):
    activerecord (3.0.10) lib/active_record/associations.rb:1553:in</code>belongs_to_counter_cache_after_create_for_form'
    activesupport (3.0.10) lib/active_support/callbacks.rb:424:in <code>_run_create_callbacks'
    activerecord (3.0.10) lib/active_record/callbacks.rb:277:in</code>create'
    activerecord (3.0.10) lib/active_record/persistence.rb:257:in <code>create_or_update'
    activerecord (3.0.10) lib/active_record/callbacks.rb:273:in</code>create_or_update'
    activesupport (3.0.10) lib/active_support/callbacks.rb:444:in <code>_run_save_callbacks'
    activerecord (3.0.10) lib/active_record/callbacks.rb:273:in</code>create_or_update'
    activerecord (3.0.10) lib/active_record/persistence.rb:60:in <code>save!'
    activerecord (3.0.10) lib/active_record/validations.rb:49:in</code>save!'
    activerecord (3.0.10) lib/active_record/attribute_methods/dirty.rb:30:in `save!'
    ...
  </code></p>
  
  <p>I'm saving Answer model which has belongs_to relation with counter cache:</p>
  
  <pre><code>belongs_to    :form, :counter_cache =&gt; true
  </code></pre>
  
  <p>Strangest thing is that after server restart first save works fine and after that I start getting such exceptions. </p>
  
  <p>Maybe someone could advise where I could start digging for a cause? </p>