Do not create useless database transaction when building `has_one` association.

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Do not create useless database transaction when building `has_one` association.
+    
+    Example:
+
+        User.has_one :profile
+        User.new.build_profile
+
+    *Bogdan Gusiev*
+
 *   :counter_cache option for `has_many` associations to support custom named counter caches.
     Fix #7993
 

activerecord/lib/active_record/associations/has_one_association.rb

@@ -28,7 +28,7 @@ def replace(record, save = true)
         # If target and record are nil, or target is equal to record,
         # we don't need to have transaction.
         if (target || record) && target != record
-          reflection.klass.transaction do
+          transaction_if(save) do
             remove_target!(options[:dependent]) if target && !target.destroyed?
 
             if record
@@ -90,6 +90,14 @@ def remove_target!(method)
         def nullify_owner_attributes(record)
           record[reflection.foreign_key] = nil
         end
+
+        def transaction_if(value)
+          if value
+            reflection.klass.transaction { yield }
+          else
+            yield
+          end
+        end
     end
   end
 end

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -206,6 +206,12 @@ def test_successful_build_association
     assert_equal account, firm.account
   end
 
+  def test_build_association_dont_create_transaction
+    assert_no_queries {
+      Firm.new.build_account
+    }
+  end
+
   def test_build_and_create_should_not_happen_within_scope
     pirate = pirates(:blackbeard)
     scoped_count = pirate.association(:foo_bulb).scope.where_values.count