don't build the reflection chain to calculate `nested?`

We know a reflection can be considered as `nested?` if the source
reflection or the through reflection are also through reflections (since
the through reflection will also add another join table).  This allows
us to avoid traversing the entire reflection tree just to calculate whether or
not there will be more join tables.

1 files changed, 9 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 0eff30f1db..2ca3206f20 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -138,6 +138,10 @@ module ActiveRecord
     #       PolymorphicReflection
     #         RuntimeReflection
     class AbstractReflection # :nodoc:
+      def through_reflection?
+        false
+      end
+
       def table_name
         klass.table_name
       end
@@ -700,6 +704,10 @@ module ActiveRecord
         @source_reflection_name = delegate_reflection.options[:source]
       end
 
+      def through_reflection?
+        true
+      end
+
       def klass
         @klass ||= delegate_reflection.compute_class(class_name)
       end
@@ -817,7 +825,7 @@ module ActiveRecord
 
       # A through association is nested if there would be more than one join table
       def nested?
-        chain.length > 2
+        source_reflection.through_reflection? || through_reflection.through_reflection?
       end
 
       # We want to use the klass from this reflection, rather than just delegate straight to