Array.count works fine locally but breaks on heroku

ruby-on-rails arrays activerecord count heroku

That's ruby issue, not rails. Locally you probably have 1.8.7, and heroku has 1.8.6. The Enumerable#count method was introduced in 1.8.7: compare http://ruby-doc.org/core-1.8.6/classes/Enumerable.html and http://ruby-doc.org/core-1.8.7/classes/Enumerable.html.