Ruby on Rails

I have a rails 3.2.21 app hosted on Heroku, using Heroku Postgres. I got the "undefined method `extend' for class `ActiveRecord::Associations::CollectionProxy'" error fired from every page on one of the web dynos. After a while the dyno was restarted and everything went back to normal. 

I have no idea what could cause such an error, any hint would be great.  

In case you need, here is a stack trace from one of the pages:

….21/lib/active_record/associations/

collection_proxy.rb:  37:in `<class:CollectionProxy>'
….21/lib/active_record/associations/

collection_proxy.rb:  36:in `<module:Associations>'
….21/lib/active_record/associations/

collection_proxy.rb:   2:in `<module:ActiveRecord>'
….21/lib/active_record/associations/

collection_proxy.rb:   1:in `<top (required)>'
…b/active_record/associations/

collection_association.rb:  25:in `initialize'
…ve_record/associations/

has_many_through_association.rb:  10:in `initialize'
…/activerecord-3.2.21/lib/active_record/

associations.rb: 160:in `new'
…/activerecord-3.2.21/lib/active_record/

associations.rb: 160:in `association'
…ib/active_record/associations/preloader/

association.rb:  18:in `run'
…ord-3.2.21/lib/active_record/associations/

preloader.rb: 132:in `block (2 levels) in preload_one'
…ord-3.2.21/lib/active_record/associations/

preloader.rb: 131:in `each'
…ord-3.2.21/lib/active_record/associations/

preloader.rb: 131:in `block in preload_one'
…ord-3.2.21/lib/active_record/associations/

preloader.rb: 130:in `each'
…ord-3.2.21/lib/active_record/associations/

preloader.rb: 130:in `preload_one'
…ord-3.2.21/lib/active_record/associations/

preloader.rb: 109:in `preload'
…/squeel/adapters/active_record/

...