Don't clean it if nothing can be cleaned.

lib/database_cleaner/active_record/transaction.rb

@@ -14,6 +14,8 @@ module DatabaseCleaner::ActiveRecord
 
 
     def clean
+      return unless connection_klass.connection.open_transactions > 0
+
       connection_klass.connection.rollback_db_transaction
 
       if connection_klass.connection.respond_to?(:decrement_open_transactions)

spec/database_cleaner/active_record/transaction_spec.rb

@@ -38,12 +38,17 @@ module DatabaseCleaner
 
       describe "#clean" do
         it "should start a transaction" do
+            connection.should_receive(:open_transactions).and_return(1)
+
             connection.stub!(:decrement_open_transactions)
 
             connection.should_receive(:rollback_db_transaction)
             Transaction.new.clean
         end
+
         it "should decrement open transactions if possible" do
+          connection.should_receive(:open_transactions).and_return(1)
+
           connection.stub!(:respond_to?).with(:decrement_open_transactions).and_return(true)
           connection.stub!(:rollback_db_transaction)
 
@@ -51,7 +56,14 @@ module DatabaseCleaner
           Transaction.new.clean
         end
 
+        it "should not try to decrement or rollback if open_transactions is 0 for whatever reason" do
+          connection.should_receive(:open_transactions).and_return(0)
+
+          Transaction.new.clean
+        end
+
         it "should decrement connection via ActiveRecord::Base if connection won't" do
+          connection.should_receive(:open_transactions).and_return(1)
           connection.stub!(:respond_to?).with(:decrement_open_transactions).and_return(false)
           connection.stub!(:rollback_db_transaction)