1 files changed, 42 insertions, 39 deletions

diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index 1109fee462..3346725f2d 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -16,20 +16,7 @@ module ActiveRecord
         end
       end
 
-      def replace(record)
-        if record
-          raise_on_type_mismatch!(record)
-          update_counters_on_replace(record)
-          set_inverse_instance(record)
-          @updated = true
-        else
-          decrement_counters
-        end
-
-        self.target = record
-      end
-
-      def target=(record)
+      def inversed_from(record)
         replace_keys(record)
         super
       end
@@ -47,26 +34,60 @@ module ActiveRecord
         @updated
       end
 
-      def decrement_counters # :nodoc:
+      def decrement_counters
         update_counters(-1)
       end
 
-      def increment_counters # :nodoc:
+      def increment_counters
         update_counters(1)
       end
 
+      def decrement_counters_before_last_save
+        if reflection.polymorphic?
+          model_was = owner.attribute_before_last_save(reflection.foreign_type).try(:constantize)
+        else
+          model_was = klass
+        end
+
+        foreign_key_was = owner.attribute_before_last_save(reflection.foreign_key)
+
+        if foreign_key_was && model_was < ActiveRecord::Base
+          update_counters_via_scope(model_was, foreign_key_was, -1)
+        end
+      end
+
+      def target_changed?
+        owner.saved_change_to_attribute?(reflection.foreign_key)
+      end
+
       private
+        def replace(record)
+          if record
+            raise_on_type_mismatch!(record)
+            set_inverse_instance(record)
+            @updated = true
+          end
+
+          replace_keys(record)
+
+          self.target = record
+        end
 
         def update_counters(by)
           if require_counter_update? && foreign_key_present?
             if target && !stale_target?
               target.increment!(reflection.counter_cache_column, by, touch: reflection.options[:touch])
             else
-              klass.update_counters(target_id, reflection.counter_cache_column => by, touch: reflection.options[:touch])
+              update_counters_via_scope(klass, owner._read_attribute(reflection.foreign_key), by)
             end
           end
         end
 
+        def update_counters_via_scope(klass, foreign_key, by)
+          scope = klass.unscoped.where!(primary_key(klass) => foreign_key)
+          scope.update_counters(reflection.counter_cache_column => by, touch: reflection.options[:touch])
+        end
+
         def find_target?
           !loaded? && foreign_key_present? && klass
         end
@@ -75,22 +96,12 @@ module ActiveRecord
           reflection.counter_cache_column && owner.persisted?
         end
 
-        def update_counters_on_replace(record)
-          if require_counter_update? && different_target?(record)
-            owner.instance_variable_set :@_after_replace_counter_called, true
-            record.increment!(reflection.counter_cache_column)
-            decrement_counters
-          end
-        end
-
-        # Checks whether record is different to the current target, without loading it
-        def different_target?(record)
-          record.id != owner._read_attribute(reflection.foreign_key)
+        def replace_keys(record)
+          owner[reflection.foreign_key] = record ? record._read_attribute(primary_key(record.class)) : nil
         end
 
-        def replace_keys(record)
-          owner[reflection.foreign_key] = record ?
-            record._read_attribute(reflection.association_primary_key(record.class)) : nil
+        def primary_key(klass)
+          reflection.association_primary_key(klass)
         end
 
         def foreign_key_present?
@@ -104,14 +115,6 @@ module ActiveRecord
           inverse && inverse.has_one?
         end
 
-        def target_id
-          if options[:primary_key]
-            owner.send(reflection.name).try(:id)
-          else
-            owner._read_attribute(reflection.foreign_key)
-          end
-        end
-
         def stale_state
           result = owner._read_attribute(reflection.foreign_key) { |n| owner.send(:missing_attribute, n, caller) }
           result && result.to_s