From 3cb0f3feed7130225cc169c2b1476acf0b11e066 Mon Sep 17 00:00:00 2001
From: Bogdan Gusiev <agresso@gmail.com>
Date: Sat, 10 Nov 2012 15:30:20 +0200
Subject: [PATCH] Do not create useless database transaction when building
 `has_one` association.

---
 activerecord/CHANGELOG.md                              |  9 +++++++++
 .../active_record/associations/has_one_association.rb  | 10 +++++++++-
 .../cases/associations/has_one_associations_test.rb    |  6 ++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 11559e3cb3..dee398177c 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,14 @@
 ## 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
 
diff --git a/activerecord/lib/active_record/associations/has_one_association.rb b/activerecord/lib/active_record/associations/has_one_association.rb
index 06bead41de..ee816d2392 100644
--- a/activerecord/lib/active_record/associations/has_one_association.rb
+++ b/activerecord/lib/active_record/associations/has_one_association.rb
@@ -28,7 +28,7 @@ module ActiveRecord
         # 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 @@ module ActiveRecord
         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
diff --git a/activerecord/test/cases/associations/has_one_associations_test.rb b/activerecord/test/cases/associations/has_one_associations_test.rb
index 2d3cb654df..ea1cfa0805 100644
--- a/activerecord/test/cases/associations/has_one_associations_test.rb
+++ b/activerecord/test/cases/associations/has_one_associations_test.rb
@@ -206,6 +206,12 @@ class HasOneAssociationsTest < ActiveRecord::TestCase
     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