Heroku nil class error while saving an object - ruby-on-rails

Question

I have an application on heroku which uses mongoid and delayed_job and i have changed the ruby version from 1.9.2 to 1.9.3 I am getting an error
undefined method `match' for nil:NilClass
while I try to save an object to mongoid data base my mongoid.yaml file looks like
production:
sessions:
default:
uri: <%= ENV['MONGOHQ_URL'] %>
options:
skip_version_check: true
safe: true
the trace is here
NoMethodError: undefined method `match' for nil:NilClass
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions/mongo_uri.rb:49:in `initialize'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions/factory.rb:103:in `new'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions/factory.rb:103:in `parse'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions/factory.rb:62:in `create_session'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions/factory.rb:43:in `default'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions.rb:109:in `default'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions.rb:378:in `__session__'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions.rb:213:in `mongo_session'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions.rb:157:in `collection'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/sessions.rb:25:in `collection'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence/operations.rb:27:in `collection'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence/operations/insert.rb:27:in `block in persist'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence/insertion.rb:25:in `block (2 levels) in prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:403:in `_run__4034630009416245289__create__685777988298500290__callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:405:in `__run_callback'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:385:in `_run_create_callbacks'
... 2 levels...
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence/insertion.rb:24:in `block in prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:403:in `_run__4034630009416245289__save__685777988298500290__callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:405:in `__run_callback'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:385:in `_run_save_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/activesupport-3.2.6/lib/active_support/callbacks.rb:81:in `run_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/callbacks.rb:114:in `run_callbacks'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence/insertion.rb:23:in `prepare'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence/operations/insert.rb:26:in `persist'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence.rb:50:in `insert'
from /app/vendor/bundle/ruby/1.9.1/gems/mongoid-3.0.13/lib/mongoid/persistence.rb:79:in `save'
from (irb):2
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.6/lib/rails/commands/console.rb:47:in `start'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.6/lib/rails/commands/console.rb:8:in `start'
from /app/vendor/bundle/ruby/1.9.1/gems/railties-3.2.6/lib/rails/commands.rb:41:in `<top (required)>'
from script/rails:6:in `require'
any one can help me out? thanks in advance

Answer 1

The error is happening because the input parameter to this method https://github.com/mongoid/mongoid/blob/v3.0.13/lib/mongoid/sessions/mongo_uri.rb#L49 is nil.
Looking further back up the call stack to find out where this string comes from (see https://github.com/mongoid/mongoid/blob/v3.0.13/lib/mongoid/sessions/factory.rb#L103) it looks like your uri config param is nil. The uri key is present but empty, so can you check your MONGOHQ_URL configuration param by running heroku config. This should have been set by the MongoHQ addon.
The Ruby 1.9.1 paths in the stack trace are not anything to worry about. See Why Do Heroku Stack Traces indicate Ruby 1.9.1 is in use?