Rails 4 has a new feature called Store you would love. You can easily use it to store a hash set which is not predefined. You can define an accessor for it and it is recommended you declare the database column used for the serialized store as a text, so there's plenty of room. The original example:
class User < ActiveRecord::Base
store :settings, accessors: [ :color, :homepage ], coder: JSON
end
u = User.new(color: 'black', homepage: '37signals.com')
u.color # Accessor stored attribute
u.settings[:country] = 'Denmark' # Any attribute, even if not specified with an accessor
# There is no difference between strings and symbols for accessing custom attributes
u.settings[:country] # => 'Denmark'
u.settings['country'] # => 'Denmark'