https://github.com/rails/rails/commit/2075f39d726cef361170218fd16421fc52bed5a8 introduced a regression in includes/preloades

by calling `read_attribute` on an association when preloading takes places, instead of using loaded records in `association.target`.

tl;dr

Records are not made properly available via `read_attribute` when preloding in simultaneous,
but value of `@loaded` is already set true, and records concatenated in `association.target` on an association object.
When `@loaded` is true we return an object of `AlreadyLoaded` in preload_for. In `AlreadyLoaded` to return preloaded
records we make wrong use of `read_attribute`, instead of `target` records.

The regression is fixed by making use of the loaded records in `association.target` when the preloading takes place.

Fixes #13437

1 files changed, 14 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/cascaded_eager_loading_test.rb b/activerecord/test/cases/associations/cascaded_eager_loading_test.rb
index 811d91f849..904d85266e 100644
--- a/activerecord/test/cases/associations/cascaded_eager_loading_test.rb
+++ b/activerecord/test/cases/associations/cascaded_eager_loading_test.rb
@@ -174,4 +174,18 @@ class CascadedEagerLoadingTest < ActiveRecord::TestCase
     sink = Vertex.all.merge!(:includes=>{:sources=>{:sources=>{:sources=>:sources}}}, :order => 'vertices.id DESC').first
     assert_equal vertices(:vertex_1), assert_no_queries { sink.sources.first.sources.first.sources.first.sources.first }
   end
+
+  def test_eager_association_loading_with_cascaded_interdependent_one_level_and_two_levels
+    authors_relation = Author.all.merge!(:includes => [:comments, {:posts => :categorizations}], :order => "authors.id")
+    assert_nothing_raised do
+      authors_relation.to_a
+    end
+    authors = authors_relation.to_a
+    assert_equal 3, authors.size
+    assert_equal 10, authors[0].comments.size
+    assert_equal 1, authors[1].comments.size
+    assert_equal 5, authors[0].posts.size
+    assert_equal 3, authors[1].posts.size
+    assert_equal 3, authors[0].posts.collect { |post| post.categorizations.size }.inject(0) { |sum, i| sum+i }
+  end
 end