Don't increment and then decrement the same counter when re-assigning a belongs_to association

[#2786 state:committed]

Signed-off-by: Santiago Pastorino <santiago@wyeworks.com>

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -22,7 +22,7 @@ module ActiveRecord
         else
           raise_on_type_mismatch(record)
 
-          if counter_cache_name && !@owner.new_record?
+          if counter_cache_name && !@owner.new_record? && record.id != @owner[@reflection.primary_key_name]
             @reflection.klass.increment_counter(counter_cache_name, record.id)
             @reflection.klass.decrement_counter(counter_cache_name, @owner[@reflection.primary_key_name]) if @owner[@reflection.primary_key_name]
           end

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -215,6 +215,10 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
 
     r1.topic = Topic.find(t2.id)
 
+    assert_no_queries do
+      r1.topic = t2
+    end
+
     assert r1.save
     assert_equal 0, Topic.find(t1.id).replies.size
     assert_equal 1, Topic.find(t2.id).replies.size