let the class cache object clean up user input

activerecord/lib/active_record/fixtures.rb

@@ -440,23 +440,35 @@ module ActiveRecord
       def initialize(class_names, config)
         @class_names = class_names.stringify_keys
         @config      = config
+
+        # Remove string values that aren't constants or subclasses of AR
+        @class_names.delete_if { |k,klass|
+          unless klass.is_a? Class
+            klass = klass.safe_constantize
+            ActiveSupport::Deprecation.warn("The ability to pass in strings as a class name will be removed in Rails 4.2, consider using the class itself instead.")
+          end
+          !insert_class(@class_names, k, klass)
+        }
       end
 
       def [](fs_name)
         @class_names.fetch(fs_name) {
           klass = default_fixture_model(fs_name, @config).safe_constantize
-
-          # We only want to deal with AR objects.
-          if klass && klass < ActiveRecord::Base
-            @class_names[fs_name] = klass
-          else
-            @class_names[fs_name] = nil
-          end
+          insert_class(@class_names, fs_name, klass)
         }
       end
 
       private
 
+      def insert_class(class_names, name, klass)
+        # We only want to deal with AR objects.
+        if klass && klass < ActiveRecord::Base
+          class_names[name] = klass
+        else
+          class_names[name] = nil
+        end
+      end
+
       def default_fixture_model(fs_name, config)
         ActiveRecord::FixtureSet.default_fixture_model_name(fs_name, config)
       end
@@ -479,7 +491,7 @@ module ActiveRecord
 
           fixture_sets = files_to_read.map do |fs_name|
             klass = class_names[fs_name]
-            conn = klass.respond_to?(:connection) ? klass.connection : connection
+            conn = klass ? klass.connection : connection
             fixtures_map[fs_name] = new( # ActiveRecord::FixtureSet.new
               conn,
               fs_name,