3 files changed, 24 insertions, 1 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index bcc7765d2e..8083a26b17 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,14 @@
 ## Rails 3.2.10 (unreleased)
 
+*   Do not create useless database transaction when building `has_one` association. [Backport #8154]
+
+    Example:
+
+        User.has_one :profile
+        User.new.build_profile
+
+    *Bogdan Gusiev*
+
 *   `AR::Base#attributes_before_type_cast` now returns unserialized values for serialized attributes.
 
     *Nikita Afanasenko*
diff --git a/activerecord/lib/active_record/associations/has_one_association.rb b/activerecord/lib/active_record/associations/has_one_association.rb
index 501ebe7c5b..56f9013d61 100644
--- a/activerecord/lib/active_record/associations/has_one_association.rb
+++ b/activerecord/lib/active_record/associations/has_one_association.rb
@@ -11,7 +11,7 @@ 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
@@ -70,6 +70,14 @@ 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 08831a42ba..31aa3788c7 100644
--- a/activerecord/test/cases/associations/has_one_associations_test.rb
+++ b/activerecord/test/cases/associations/has_one_associations_test.rb
@@ -173,6 +173,12 @@ 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).scoped.where_values.count