Fix preload with nested associations

When the middle association doesn't have any records and the inner
association is not an empty scope the owner will be `nil` so we can't
try to reset the inverse association.

activerecord/lib/active_record/associations/preloader/through_association.rb

@@ -14,6 +14,7 @@ def run(preloader)
 
           owners.each do |owner|
             through_records = Array(owner.association(through_reflection.name).target)
+
             if already_loaded
               if source_type = reflection.options[:source_type]
                 through_records = through_records.select do |record|
@@ -23,17 +24,20 @@ def run(preloader)
             else
               owner.association(through_reflection.name).reset if through_scope
             end
+
             result = through_records.flat_map do |record|
               record.association(source_reflection.name).target
             end
+
             result.compact!
             result.sort_by! { |rhs| preload_index[rhs] } if scope.order_values.any?
             result.uniq! if scope.distinct_value
             associate_records_to_owner(owner, result)
           end
+
           unless scope.empty_scope?
             middle_records.each do |owner|
-              owner.association(source_reflection.name).reset
+              owner.association(source_reflection.name).reset if owner
             end
           end
         end

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -55,6 +55,15 @@ def test_marshal_dump
     assert_equal preloaded, Marshal.load(Marshal.dump(preloaded))
   end
 
+  def test_preload_with_nested_association
+    posts = Post.preload(:author, :author_favorites).to_a
+
+    assert_no_queries do
+      posts.each(&:author)
+      posts.each(&:author_favorites)
+    end
+  end
+
   def test_preload_sti_rhs_class
     developers = Developer.includes(:firms).all.to_a
     assert_no_queries do

activerecord/test/models/author.rb

@@ -217,6 +217,8 @@ def self.destroyed_author_address_ids
 end
 
 class AuthorFavorite < ActiveRecord::Base
+  default_scope { order(id: :asc) }
+
   belongs_to :author
   belongs_to :favorite_author, class_name: "Author"
 end