Fix exception if old and new targets are both nil. Fixes #1471.

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -37,7 +37,7 @@ def update_counters(record)
         # Checks whether record is different to the current target, without loading it
         def different_target?(record)
           record.nil? && owner[reflection.foreign_key] ||
-          record.id   != owner[reflection.foreign_key]
+          record && record.id != owner[reflection.foreign_key]
         end
 
         def replace_keys(record)

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -681,4 +681,13 @@ def test_should_set_foreign_key_on_create_association!
     firm = client.create_firm! :name => "baa"
     assert_equal firm.id, client.client_of
   end
+
+  def test_self_referential_belongs_to_with_counter_cache_assigning_nil
+    comment = Comment.create! :post => posts(:thinking), :body => "fuu"
+    comment.parent = nil
+    comment.save!
+
+    assert_equal nil, comment.reload.parent
+    assert_equal 0, comments(:greetings).reload.children_count
+  end
 end

activerecord/test/models/comment.rb

@@ -7,10 +7,13 @@ class Comment < ActiveRecord::Base
               :joins => :post,
               :conditions => { "posts.author_id" => 1 }
   scope :created
-  
+
   belongs_to :post, :counter_cache => true
   has_many :ratings
 
+  has_many :children, :class_name => 'Comment', :foreign_key => :parent_id
+  belongs_to :parent, :class_name => 'Comment', :counter_cache => :children_count
+
   def self.what_are_you
     'a comment...'
   end

activerecord/test/schema/schema.rb

@@ -155,6 +155,7 @@ def create_table(*args, &block)
     end
     t.string  :type
     t.integer :taggings_count, :default => 0
+    t.integer :children_count, :default => 0
   end
 
   create_table :companies, :force => true do |t|