Ran into this one today at LP while attempting to get one of our applications out on Heroku. We are using a fairly old gem called spatial_adapter which is no longer under active development.
We were failing to ‘get it up’ (ha!) due to it barfing while setting up the spatial_adapter railtie.
The stack trace we were getting was
/app/vendor/bundle/ruby/1.9.1/bundler/gems/spatial_adapter-d64ac1c03c83/lib/spatial_adapter/railtie.rb:5:in `block (2 levels) in <class:Railtie>': undefined method `' for nil:NilClass (NoMethodError)
I forked the repo and updated the railtie to grab the configuration in the correct manner
#Busted ActiveRecord::Base.configurations[Rails.env]['adapter'] #Works Rails.configuration.database_configuration[Rails.env]['adapter']
Fixed! Although I don’t know how many others will run into the same problem :)