hstore_accessor gem. It provides discrete fields to directly access values in hstore columns to ActiveRecord.
A blog post on using an hstore column for storing hashes with arbitrary keys. His approach is different from the one used in hstoreaccessor gem. Aly explains how to store hashes that may have different keys for each record, while hstoreaccessor gem implies that hashed in all the records have the same keys.
Next, a very practical article on using Postgres array columns in Rails 4 by Plataformatec. They explain how to use an array column for storing tags.
Finally, a post by Thoughtbot on why PostgreSQL may sometimes not use indexes in SELECT queries.