It depends on use cases and you are many choices. But, in my opinion, it's better to avoid using two kinds of db together when possible.
If you are not using PostgreSQL and no heavy queries needed for those attributes, ActiveRecord serialize
can help. It utilize a text field to stored serialized hash and arrays. Nesting acceptable, speed slower.
If all of the attributes are one level key/value pair, say, warranty: 18 months, vendor: 123, then "hstore" is a good choice, built-in in PostgreSQL.
"hstore" is flexible and fast, but not suitable for nested hash. And there is a great gem dealing with it: https://github.com/diogob/activerecord-postgres-hstore
If nested hash, PostgreSQL "JSON" fieldtype may help. http://www.postgresql.org/docs/devel/static/functions-json.html It's new and I havn't used that before, just heard it's more stable and practical.
I think those choices should be enough to solve your problem. If they still can't do, maybe you can consider MongoDB.