Require explicit counter_cache option for has_many

Previously has_many associations assumed a counter_cache was to be used
based on the presence of an appropriately named column. This is
inconsistent, since the inverse belongs_to association will not make
this assumption. See issues #19042 #8446.
This commit checks for the presence of the counter_cache key in the
options of either the has_many or belongs_to association as well as
ensuring that the *_count column is present.

activerecord/lib/active_record/associations/has_many_association.rb

@@ -80,8 +80,15 @@ def count_records
           [association_scope.limit_value, count].compact.min
         end
 
+
+        # Returns whether a counter cache should be used for this association.
+        #
+        # The counter_cache option must be given on either the owner or inverse
+        # association, and the column must be present on the owner.
         def has_cached_counter?(reflection = reflection())
-          owner.attribute_present?(cached_counter_attribute_name(reflection))
+          if reflection.options[:counter_cache] || (inverse = inverse_which_updates_counter_cache(reflection)) && inverse.options[:counter_cache]
+            owner.attribute_present?(cached_counter_attribute_name(reflection))
+          end
         end
 
         def cached_counter_attribute_name(reflection = reflection())

activerecord/test/models/categorization.rb

@@ -1,6 +1,6 @@
 class Categorization < ActiveRecord::Base
   belongs_to :post
-  belongs_to :category
+  belongs_to :category, counter_cache: true
   belongs_to :named_category, :class_name => 'Category', :foreign_key => :named_category_name, :primary_key => :name
   belongs_to :author
 

activerecord/test/models/post.rb

@@ -98,11 +98,11 @@ def add_joins_and_select
     end
   end
 
-  has_many :taggings_with_delete_all, :class_name => 'Tagging', :as => :taggable, :dependent => :delete_all
-  has_many :taggings_with_destroy, :class_name => 'Tagging', :as => :taggable, :dependent => :destroy
+  has_many :taggings_with_delete_all, :class_name => 'Tagging', :as => :taggable, :dependent => :delete_all, counter_cache: :taggings_with_delete_all_count
+  has_many :taggings_with_destroy, :class_name => 'Tagging', :as => :taggable, :dependent => :destroy, counter_cache: :taggings_with_destroy_count
 
-  has_many :tags_with_destroy, :through => :taggings, :source => :tag, :dependent => :destroy
-  has_many :tags_with_nullify, :through => :taggings, :source => :tag, :dependent => :nullify
+  has_many :tags_with_destroy, :through => :taggings, :source => :tag, :dependent => :destroy, counter_cache: :tags_with_destroy_count
+  has_many :tags_with_nullify, :through => :taggings, :source => :tag, :dependent => :nullify, counter_cache: :tags_with_nullify_count
 
   has_many :misc_tags, -> { where :tags => { :name => 'Misc' } }, :through => :taggings, :source => :tag
   has_many :funky_tags, :through => :taggings, :source => :tag