stop relying on side effects of const_missing

activerecord/lib/active_record/relation/delegation.rb

@@ -77,12 +77,6 @@ module ActiveRecord
         relation_class_for(klass).new(klass, *args)
       end
 
-      # This doesn't have to be thread-safe. relation_class_for guarantees that this will only be
-      # called exactly once for a given const name.
-      def const_missing(name)
-        const_set(name, Class.new(self) { include ClassSpecificRelation })
-      end
-
       private
       # Cache the constants in @@subclasses because looking them up via const_get
       # make instantiation significantly slower.
@@ -92,7 +86,13 @@ module ActiveRecord
           # This hash is keyed by klass.name to avoid memory leaks in development mode
           my_cache.compute_if_absent(klass_name) do
             # Cache#compute_if_absent guarantees that the block will only executed once for the given klass_name
-            const_get("#{name.gsub('::', '_')}_#{klass_name.gsub('::', '_')}", false)
+            subclass_name = "#{name.gsub('::', '_')}_#{klass_name.gsub('::', '_')}"
+
+            if const_defined?(subclass_name)
+              const_get(subclass_name)
+            else
+              const_set(subclass_name, Class.new(self) { include ClassSpecificRelation })
+            end
           end
         else
           ActiveRecord::Relation

activerecord/test/cases/relation_test.rb

@@ -111,6 +111,12 @@ module ActiveRecord
       assert_equal({}, relation.scope_for_create)
     end
 
+    def test_bad_constants_raise_errors
+      assert_raises(NameError) do
+        ActiveRecord::Relation::HelloWorld
+      end
+    end
+
     def test_empty_eager_loading?
       relation = Relation.new FakeKlass, :b
       assert !relation.eager_loading?