Deprecate automatic counter caches on has_many :through

Reliant on https://github.com/rails/rails/pull/15747 but pulled to a
separate PR to reduce noise. `has_many :through` associations have the
undocumented behavior of automatically detecting counter caches.
However, the way in which it does so is inconsistent with counter caches
everywhere else, and doesn't actually work consistently.

As with normal `has_many` associations, the user should specify the
counter cache on the `belongs_to`, if they'd like it updated.

2 files changed, 23 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/has_many_association.rb b/activerecord/lib/active_record/associations/has_many_association.rb
index 477888228d..453615ba87 100644
--- a/activerecord/lib/active_record/associations/has_many_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_association.rb
@@ -80,10 +80,14 @@ module ActiveRecord
         end
 
         def update_counter(difference, reflection = reflection())
+          update_counter_in_database(difference, reflection)
+          update_counter_in_memory(difference, reflection)
+        end
+
+        def update_counter_in_database(difference, reflection = reflection())
           if has_cached_counter?(reflection)
             counter = cached_counter_attribute_name(reflection)
             owner.class.update_counters(owner.id, counter => difference)
-            update_counter_in_memory(difference, reflection)
           end
         end
 
@@ -107,6 +111,10 @@ module ActiveRecord
         # Hence this method.
         def inverse_updates_counter_cache?(reflection = reflection())
           counter_name = cached_counter_attribute_name(reflection)
+          inverse_updates_counter_named?(counter_name, reflection)
+        end
+
+        def inverse_updates_counter_named?(counter_name, reflection = reflection())
           reflection.klass._reflections.values.any? { |inverse_reflection|
             :belongs_to == inverse_reflection.macro &&
             inverse_reflection.counter_cache_column == counter_name
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index af38f2f6dd..007e3bc555 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -63,6 +63,15 @@ module ActiveRecord
         end
 
         save_through_record(record)
+        if has_cached_counter? && !through_reflection_updates_counter_cache?
+          ActiveSupport::Deprecation.warn(<<-MESSAGE.strip_heredoc)
+            Automatic updating of counter caches on through associations has been
+            deprecated, and will be removed in Rails 5.0. Instead, please set the
+            appropriate counter_cache options on the has_many and belongs_to for
+            your associations to #{through_reflection.name}.
+          MESSAGE
+          update_counter_in_database(1)
+        end
         record
       end
 
@@ -217,6 +226,11 @@ module ActiveRecord
         def invertible_for?(record)
           false
         end
+
+        def through_reflection_updates_counter_cache?
+          counter_name = cached_counter_attribute_name
+          inverse_updates_counter_named?(counter_name, through_reflection)
+        end
     end
   end
 end