select proper default database. fix transactional cleaner.

lib/database_cleaner/sequel/base.rb

@@ -13,9 +13,10 @@ def db=(desired_db)
       end
 
       def db
-        @db || :default
+        return @db if @db && @db != :default
+        raise "As you have more than one active sequel database you have to specify the one to use manually!" if ::Sequel::DATABASES.count > 1 
+        ::Sequel::DATABASES.first
       end
-
     end
   end
 end

lib/database_cleaner/sequel/transaction.rb

@@ -5,11 +5,15 @@ class Transaction
       include ::DatabaseCleaner::Sequel::Base
 
       def start
-        db.begin_transaction
+        @transactions ||= []
+        db.send(:add_transaction)
+        @transactions << db.send(:begin_transaction, db)
       end
 
       def clean
-        db.rollback_transaction
+        transaction = @transactions.pop
+        db.send(:rollback_transaction, transaction)
+        db.send(:remove_transaction, transaction)
       end
     end
   end