Merge pull request rails#50129 from MaxLap/fix_preload_concat

Fix: Preloader should no longer overwrites built records

Author: tenderlove

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

@@ -245,7 +245,8 @@ def associate_records_to_owner(owner, records)
             association = owner.association(reflection.name)
 
             if reflection.collection?
-              association.target = records
+              association.loaded!
+              association.target.concat(records)
             else
               association.target = records.first
             end

activerecord/test/cases/associations_test.rb

@@ -1452,6 +1452,51 @@ def test_preloads_belongs_to_a_composite_primary_key_model_through_id_attribute
     assert_match(expectation, preload_sql)
     assert_equal order, loaded_order_agreement.order
   end
+
+  def test_preload_keeps_built_has_many_records_no_ops
+    post = Post.new
+    comment = post.comments.build
+
+    assert_no_queries do
+      ActiveRecord::Associations::Preloader.new(records: [post], associations: :comments).call
+
+      assert_equal [comment], post.comments.to_a
+    end
+  end
+
+  def test_preload_keeps_built_has_many_records_after_query
+    post = posts(:welcome)
+    comment = post.comments.build
+
+    assert_queries(1) do
+      ActiveRecord::Associations::Preloader.new(records: [post], associations: :comments).call
+
+      assert_includes post.comments.to_a, comment
+    end
+  end
+
+
+  def test_preload_keeps_built_belongs_to_records_no_ops
+    post = Post.new
+    author = post.build_author
+
+    assert_no_queries do
+      ActiveRecord::Associations::Preloader.new(records: [post], associations: :author).call
+
+      assert_same author, post.author
+    end
+  end
+
+  def test_preload_keeps_built_belongs_to_records_after_query
+    post = posts(:welcome)
+    author = post.build_author
+
+    assert_no_queries do
+      ActiveRecord::Associations::Preloader.new(records: [post], associations: :author).call
+
+      assert_same author, post.author
+    end
+  end
 end
 
 class GeneratedMethodsTest < ActiveRecord::TestCase