2 files changed, 15 insertions, 3 deletions
diff --git a/activerecord/lib/active_record/fixtures.rb b/activerecord/lib/active_record/fixtures.rb
index 17e7c828b9..deb7d7d06d 100644
--- a/activerecord/lib/active_record/fixtures.rb
+++ b/activerecord/lib/active_record/fixtures.rb
@@ -875,9 +875,7 @@ module ActiveRecord
       self.pre_loaded_fixtures = false
       self.config = ActiveRecord::Base
 
-      self.fixture_class_names = Hash.new do |h, fixture_set_name|
-        h[fixture_set_name] = ActiveRecord::FixtureSet.default_fixture_model_name(fixture_set_name, self.config)
-      end
+      self.fixture_class_names = {}
 
       silence_warnings do
         define_singleton_method :use_transactional_tests do
diff --git a/activerecord/test/cases/fixtures_test.rb b/activerecord/test/cases/fixtures_test.rb
index a0eaa66e94..f30ed4fcc8 100644
--- a/activerecord/test/cases/fixtures_test.rb
+++ b/activerecord/test/cases/fixtures_test.rb
@@ -955,3 +955,17 @@ class FixturesWithAbstractBelongsTo < ActiveRecord::TestCase
     assert_equal pirates(:blackbeard), doubloons(:blackbeards_doubloon).pirate
   end
 end
+
+class FixtureClassNamesTest < ActiveRecord::TestCase
+  def setup
+    @saved_cache = self.fixture_class_names.dup
+  end
+
+  def teardown
+    self.fixture_class_names.replace(@saved_cache)
+  end
+
+  test "fixture_class_names returns nil for unregistered identifier" do
+    assert_nil self.fixture_class_names['unregistered_identifier']
+  end
+end