Fix attaching many uploaded files one at a time

Closes #36806.

activestorage/lib/active_storage/attached/changes/delete_many.rb

@@ -8,6 +8,10 @@ def initialize(name, record)
       @name, @record = name, record
     end
 
+    def attachables
+      []
+    end
+
     def attachments
       ActiveStorage::Attachment.none
     end

activestorage/lib/active_storage/attached/many.rb

@@ -31,7 +31,7 @@ def attach(*attachables)
       if record.persisted? && !record.changed?
         record.update(name => blobs + attachables.flatten)
       else
-        record.public_send("#{name}=", blobs + attachables.flatten)
+        record.public_send("#{name}=", (change&.attachables || blobs) + attachables.flatten)
       end
     end
 

activestorage/test/models/attached/many_test.rb

@@ -109,6 +109,27 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
     assert_equal "video.mp4", @user.highlights.second.filename.to_s
   end
 
+  test "attaching new blobs from uploaded files to an existing, changed record one at a time" do
+    @user.name = "Tina"
+    assert @user.changed?
+
+    @user.highlights.attach fixture_file_upload("racecar.jpg")
+    @user.highlights.attach fixture_file_upload("video.mp4")
+    assert_equal "racecar.jpg", @user.highlights.first.filename.to_s
+    assert_equal "video.mp4", @user.highlights.second.filename.to_s
+    assert_not @user.highlights.first.persisted?
+    assert_not @user.highlights.second.persisted?
+    assert @user.will_save_change_to_name?
+    assert_not ActiveStorage::Blob.service.exist?(@user.highlights.first.key)
+    assert_not ActiveStorage::Blob.service.exist?(@user.highlights.second.key)
+
+    @user.save!
+    assert_equal "racecar.jpg", @user.highlights.reload.first.filename.to_s
+    assert_equal "video.mp4", @user.highlights.second.filename.to_s
+    assert ActiveStorage::Blob.service.exist?(@user.highlights.first.key)
+    assert ActiveStorage::Blob.service.exist?(@user.highlights.second.key)
+  end
+
   test "attaching existing blobs to an existing record one at a time" do
     @user.highlights.attach create_blob(filename: "funky.jpg")
     @user.highlights.attach create_blob(filename: "town.jpg")