fix multiple hash preloads. Fixes #14994

activerecord/lib/active_record/associations/preloader.rb

@@ -112,13 +112,14 @@ def preloaders_on(association, records, scope)
       end
 
       def preloaders_for_hash(association, records, scope)
-        parent, child = association.to_a.first # hash should only be of length 1
+        association.flat_map { |parent, child|
+          loaders = preloaders_for_one parent, records, scope
 
-        loaders = preloaders_for_one parent, records, scope
-
-        recs = loaders.flat_map(&:preloaded_records).uniq
-        loaders.concat Array.wrap(child).flat_map { |assoc|
-          preloaders_on assoc, recs, scope
+          recs = loaders.flat_map(&:preloaded_records).uniq
+          loaders.concat Array.wrap(child).flat_map { |assoc|
+            preloaders_on assoc, recs, scope
+          }
+          loaders
         }
       end
 

activerecord/test/cases/relations_test.rb

@@ -508,6 +508,13 @@ def test_find_with_preloaded_associations
     end
   end
 
+  def test_deep_preload
+    post = Post.preload(author: :posts, comments: :post).first
+
+    assert_predicate post.author.association(:posts), :loaded?
+    assert_predicate post.comments.first.association(:post), :loaded?
+  end
+
   def test_preload_applies_to_all_chained_preloaded_scopes
     assert_queries(3) do
       post = Post.with_comments.with_tags.first