FixtureSet.fixture_class_names should have no default value

Look at `TestFixtures.set_fixture_class`. As documented, it
accepts a mapping of fixture identifiers (string or symbol) to Classes
(the model classes that implement the named fixture).

Look now at the initialization of `TestFixtures.fixture_class_names`.
It defines a Hash, which will return a string by default (where the
string is the estimated class name of the given fixture identifier).

Now look at TestFixtures.load_fixtures. It calls `FixtureSet.create_fixtures`,
passing in the mapping of `fixture_class_names`.

Following this on to `FixtureSet.create_fixtures`, this instantiates a
`FixtureSet::ClassCache`, passing in the map of class names.

`ClassCache`, in turn, calls `insert_class` for each value in the cache.
(Recall that `set_fixture_class` puts Class objects in there, while the
default proc for the mapping puts String objects.)

Look finally at `insert_class`. If the value is present, it checks to
see if the value is a subclass of `AR::Base`. Fair enough...but wait!
What if the value is a String? You get an exception, because a String
instance cannot be compared with a Class.

Judging from the implementation, it seems like the expected behavior
here is for `fixture_class_names` to have no default proc. Look-ups are
supposed to happen via `ClassCache`, with `fixture_class_names` existing
solely as a repository for explicitly-registered class mappings.

That is what this change does.

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