Return `through_scope` only if the scope is not empty scope

Related 2b5f5cdd7c1d95716de6a206b6d09ccbb006dc17.

If `through_scope` is empty scope, it is unnecessary to merge it.
And also, comparing relations is a little expensive (will cause
`build_arel`). It is enough to use `empty_scope?` to determine whether
empty scope.

1 files changed, 2 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 7b37ec2f41..26bcdcbcc4 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -67,9 +67,7 @@ module ActiveRecord
             id_map
           end
 
-          def reset_association(owners, association_name, through_scope)
-            should_reset = through_scope != through_reflection.klass.unscoped
-
+          def reset_association(owners, association_name, should_reset)
             # Don't cache the association - we would only be caching a subset
             if should_reset
               owners.each { |owner|
@@ -112,7 +110,7 @@ module ActiveRecord
               end
             end
 
-            scope
+            scope unless scope.empty_scope?
           end
       end
     end