Merge pull request rails#17811 from mrnugget/fixes_nested_association_preloading

Adds preloaded_records method to NullPreloader - fixes rails#16070

Author: spastorino

activerecord/lib/active_record/associations/preloader.rb

@@ -178,6 +178,7 @@ def preloaded_records
       class NullPreloader
         def self.new(klass, owners, reflection, preload_scope); self; end
         def self.run(preloader); end
+        def self.preloaded_records; []; end
       end
 
       def preloader_for(reflection, owners, rhs_klass)

activerecord/test/cases/associations/eager_test.rb

@@ -270,6 +270,14 @@ def test_nested_loading_does_not_raise_exception_when_association_does_not_exist
     end
   end
 
+  def test_three_level_nested_preloading_does_not_raise_exception_when_association_does_not_exist
+    post_id = Comment.where(author_id: nil).where.not(post_id: nil).first.post_id
+
+    assert_nothing_raised do
+      Post.preload(:comments => [{:author => :essays}]).find(post_id)
+    end
+  end
+
   def test_nested_loading_through_has_one_association
     aa = AuthorAddress.all.merge!(:includes => {:author => :posts}).find(author_addresses(:david_address).id)
     assert_equal aa.author.posts.count, aa.author.posts.length