DATABASE_URL allows omission of database.yml

activerecord/lib/active_record/railtie.rb

@@ -68,7 +68,9 @@ class Railtie < Rails::Railtie
     # and then establishes the connection.
     initializer "active_record.initialize_database" do |app|
       ActiveSupport.on_load(:active_record) do
-        self.configurations = app.config.database_configuration
+        unless ENV['DATABASE_URL']
+          self.configurations = app.config.database_configuration
+        end
         establish_connection
       end
     end