This test does not test anything that happens in the real world. If you

recreate the models without mucking with internal caches of the relation
objects, then the test fails.

For example:

class Man < ActiveRecord::Base
  has_many :interests
end

class Interest < ActiveRecord::Base
  belongs_to :man
end

Then do this test:

def test_validate_presence_of_parent_fails_without_inverse_of
  repair_validations(Interest) do
    Interest.validates_presence_of(:man)
    assert_no_difference ['Man.count', 'Interest.count'] do
      man = Man.create(:name => 'John',
                       :interests_attributes => [{:topic=>'Cars'}, {:topic=>'Sports'}])
      assert_not_predicate man.errors[:"interests.man"], :empty?
    end
  end
end

The test will fail.  This is a bad test, so I am removing it.

activerecord/lib/active_record/reflection.rb

@@ -310,13 +310,6 @@ def inverse_of
         @inverse_of ||= klass.reflect_on_association inverse_name
       end
 
-      # Clears the cached value of +@inverse_of+ on this object. This will
-      # not remove the :inverse_of option however, so future calls on the
-      # +inverse_of+ will have to recompute the inverse.
-      def clear_inverse_of_cache!
-        @inverse_of = nil
-      end
-
       def polymorphic_inverse_of(associated_class)
         if has_inverse?
           if inverse_relationship = associated_class.reflect_on_association(options[:inverse_of])

activerecord/test/cases/nested_attributes_test.rb

@@ -797,26 +797,6 @@ def test_validate_presence_of_parent_works_with_inverse_of
     end
   end
 
-  def test_validate_presence_of_parent_fails_without_inverse_of
-    Man.accepts_nested_attributes_for(:interests)
-    Man.reflect_on_association(:interests).options.delete(:inverse_of)
-    Man.reflect_on_association(:interests).clear_inverse_of_cache!
-    Interest.reflect_on_association(:man).options.delete(:inverse_of)
-    Interest.reflect_on_association(:man).clear_inverse_of_cache!
-
-    repair_validations(Interest) do
-      Interest.validates_presence_of(:man)
-      assert_no_difference ['Man.count', 'Interest.count'] do
-        man = Man.create(:name => 'John',
-                         :interests_attributes => [{:topic=>'Cars'}, {:topic=>'Sports'}])
-        assert_not_predicate man.errors[:"interests.man"], :empty?
-      end
-    end
-  ensure
-    Man.reflect_on_association(:interests).options[:inverse_of] = :man
-    Interest.reflect_on_association(:man).options[:inverse_of]  = :interests
-  end
-
   def test_can_use_symbols_as_object_identifier
     @pirate.attributes = { :parrots_attributes => { :foo => { :name => 'Lovely Day' }, :bar => { :name => 'Blown Away' } } }
     assert_nothing_raised(NoMethodError) { @pirate.save! }