spec

# File activerecord/lib/active_record/connection_adapters/connection_specification.rb, line 158
        def spec(config)
          pool_name = config if config.is_a?(Symbol)

          spec = resolve(config, pool_name).symbolize_keys

          raise(AdapterNotSpecified, "database configuration does not specify adapter") unless spec.key?(:adapter)

          # Require the adapter itself and give useful feedback about
          #   1. Missing adapter gems and
          #   2. Adapter gems' missing dependencies.
          path_to_adapter = "active_record/connection_adapters/#{spec[:adapter]}_adapter"
          begin
            require path_to_adapter
          rescue LoadError => e
            # We couldn't require the adapter itself. Raise an exception that
            # points out config typos and missing gems.
            if e.path == path_to_adapter
              # We can assume that a non-builtin adapter was specified, so it's
              # either misspelled or missing from Gemfile.
              raise LoadError, "Could not load the '#{spec[:adapter]}' Active Record adapter. Ensure that the adapter is spelled correctly in config/database.yml and that you've added the necessary adapter gem to your Gemfile.", e.backtrace

            # Bubbled up from the adapter require. Prefix the exception message
            # with some guidance about how to address it and reraise.
            else
              raise LoadError, "Error loading the '#{spec[:adapter]}' Active Record adapter. Missing a gem it depends on? #{e.message}", e.backtrace
            end
          end

          adapter_method = "#{spec[:adapter]}_connection"

          unless ActiveRecord::Base.respond_to?(adapter_method)
            raise AdapterNotFound, "database configuration specifies nonexistent #{spec.config[:adapter]} adapter"
          end

          ConnectionSpecification.new(spec.delete(:name) || "primary", spec, adapter_method)
        end