Heroku Invalid database_url with mongohq - ruby-on-rails

Question

Im trying to deploy my application in heroku. My app uses mongohq for the database. I have added the respective add-on to my app.
When deploying the app i get this error.
Cleaning up the bundler cache.
-----> Writing config/database.yml to read from DATABASE_URL
-----> Preparing app for Rails asset pipeline
Running: rake assets:precompile
** Notice: The native BSON extension was not loaded. **
For optimal performance, use of the BSON extension is recommended.
To enable the extension make sure ENV['BSON_EXT_DISABLED'] is not set
and run the following command:
gem install bson_ext
If you continue to receive this message after installing, make sure that
the bson_ext gem is in your load path.
rake aborted!
Invalid DATABASE_URL
(erb):9:in `rescue in <main>'
(erb):6:in `<main>'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application/configuration.rb:106:in `database_configuration'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/railtie.rb:174:in `block (2 levels) in <class:Railtie>'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/lazy_load_hooks.rb:28:in `block in on_load'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/lazy_load_hooks.rb:27:in `each'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/lazy_load_hooks.rb:27:in `on_load'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/railtie.rb:173:in `block in <class:Railtie>'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:30:in `instance_exec'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:30:in `run'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:55:in `block in run_initializers'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:54:in `run_initializers'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:215:in `initialize!'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/railtie/configurable.rb:30:in `method_missing'
/tmp/build_3o2y20a7ewhl/config/environment.rb:5:in `<top (required)>'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:189:in `require_environment!'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:249:in `block in run_tasks_blocks'
/tmp/build_3o2y20a7ewhl/vendor/bundle/ruby/2.0.0/gems/sprockets-rails-2.0.0/lib/sprockets/rails/task.rb:54:in `block (2 levels) in define'
Tasks: TOP => environment
(See full trace by running task with --trace)
!
! Precompiling assets failed.
!
! Push rejected, failed to compile Ruby/Rails app
It looks like is trying to write the DATABASE_URL in database.yml but i don't use one. I just use monogoid.yml and set the proper ENV var MONGOHQ_URL.
Do i need to setup any configuration so heroku can detect i'm using mongo as my database?
EDIT:
My mongoid.yml file looks like this.
production:
sessions:
default:
uri: <%= ENV['MONGOHQ_URL'] %>
I have read that article, i'm following those steps.

Answer 1

In case anyone is going through the same, i had in my application.rb a reference to active record that i needed to take away:
#require "active_record/railtie"
Also double check your environments files, in some cases you could have some active_record configuration code.
Hope it helps.