Это работает:
>> Card.find(2)
=> #<Card id: 2, original_card_id: 46, level: 2, expa: 1300, user_id: nil, created_at: nil>

>> OriginalCard.find(46).cards.find(:first)
=> #<Card id: 2, original_card_id: 46, level: 2, expa: 1300, user_id: nil, created_at: nil>

А в обратную сторону - фиг.
>> OriginalCard.find(46).cards.find(:first).original_card
#(или >> Card.find(2).original_card - результат одинаковый.. )#

NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.each
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:70:in `set_association_single_records'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:68:in `each'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:68:in `set_association_single_records'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:228:in `preload_belongs_to_association'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:215:in `each'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:215:in `preload_belongs_to_association'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:40:in `send'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:40:in `preload_one_association'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:38:in `each'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:38:in `preload_one_association'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:17:in `preload_associations'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:16:in `preload_associations'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:16:in `each'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/association_preload.rb:16:in `preload_associations'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/base.rb:1316:in `find_every'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/base.rb:1349:in `find_one'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/base.rb:1335:in `find_from_ids'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/base.rb:519:in `find'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/associations/belongs_to_association.rb:44:in `find_target'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:186:in `load_target'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:99:in `reload'
        from /home/docent/rails-apps/repo_klanz/klanz.git/klanz/vendor/rails/activerecord/lib/active_record/associations.rb:1069:in `original_card'
        from (irb):12