Fix clash with methods named fixture in test cases

Fix: rails#51213 (comment)

activerecord/lib/active_record/test_fixtures.rb

@@ -255,7 +255,7 @@ def load_instances?
 
       def method_missing(method, ...)
         if fixture_sets.key?(method.name)
-          fixture(method, ...)
+          _active_record_fixture(method, ...)
         else
           super
         end
@@ -269,13 +269,14 @@ def respond_to_missing?(method, include_private = false)
         end
       end
 
-      def fixture(fixture_set_name, *fixture_names)
+      def _active_record_fixture(fixture_set_name, *fixture_names)
         if fs_name = fixture_sets[fixture_set_name.name]
           access_fixture(fs_name, *fixture_names)
         else
           raise StandardError, "No fixture set named '#{fixture_set_name.inspect}'"
         end
       end
+      alias_method :fixture, :_active_record_fixture
 
       def access_fixture(fs_name, *fixture_names)
         force_reload = fixture_names.pop if fixture_names.last == true || fixture_names.last == :reload

activerecord/test/cases/fixtures_test.rb

@@ -604,6 +604,31 @@ def test_fixtures
   ensure
     ENV["DATABASE_URL"] = db_url_tmp
   end
+
+  def test_fixture_method_and_private_alias
+    assert_equal "The First Topic", topics(:first).title
+    assert_equal "The First Topic", fixture(:topics, :first).title
+    assert_equal "The First Topic", _active_record_fixture(:topics, :first).title
+  end
+
+  def test_fixture_method_does_not_clash_with_a_test_case_method
+    test_case = Class.new(ActiveRecord::TestCase) do
+      fixtures :accounts
+
+      def test_fixtures
+        assert accounts(:signals37)
+      end
+
+      private
+        def fixture
+          Account.new
+        end
+    end
+
+    result = test_case.new(:test_fixtures).run
+
+    assert_predicate result, :passed?, "Expected #{result.name} to pass:\n#{result}"
+  end
 end
 
 class HasManyThroughFixture < ActiveRecord::TestCase