Abstract a bit more into SingularAssociation

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -3,8 +3,7 @@ module ActiveRecord
   module Associations
     class BelongsToAssociation < SingularAssociation #:nodoc:
       def replace(record)
-        record = record.target if AssociationProxy === record
-        raise_on_type_mismatch(record) if record
+        record = check_record(record)
 
         update_counters(record)
         replace_keys(record)

activerecord/lib/active_record/associations/has_one_association.rb

@@ -3,8 +3,7 @@ module ActiveRecord
   module Associations
     class HasOneAssociation < SingularAssociation #:nodoc:
       def replace(record, save = true)
-        record = record.target if AssociationProxy === record
-        raise_on_type_mismatch(record) unless record.nil?
+        record = check_record(record)
         load_target
 
         @reflection.klass.transaction do

activerecord/lib/active_record/associations/singular_association.rb

@@ -26,6 +26,12 @@ def replace(record)
         def set_new_record(record)
           replace(record)
         end
+
+        def check_record(record)
+          record = record.target if AssociationProxy === record
+          raise_on_type_mismatch(record) if record
+          record
+        end
     end
   end
 end