replace

# File activerecord/lib/active_record/associations/belongs_to_association.rb, line 12
      def replace(record)
        counter_cache_name = @reflection.counter_cache_column

        if record.nil?
          if counter_cache_name && !@owner.new_record?
            @reflection.klass.decrement_counter(counter_cache_name, @owner[@reflection.primary_key_name]) if @owner[@reflection.primary_key_name]
          end

          @target = @owner[@reflection.primary_key_name] = nil
        else
          raise_on_type_mismatch(record)

          if counter_cache_name && !@owner.new_record?
            @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

          @target = (AssociationProxy === record ? record.target : record)
          @owner[@reflection.primary_key_name] = record.id unless record.new_record?
          @updated = true
        end

        loaded
        record
      end