ActiveRecord and Serialization Revisited

2013.04.03

Not long after posting about how ActiveRecord serializes a Ruby object, a friend showed me the serialize method. Although there are likely cases where converting a Ruby object to YAML has its uses, ActiveRecord in fact supports storing objects within a database for later retrieval.

For example, assume we have a User class with a preferences attribute. (Note: the following example comes from this page in the Ruby on Rails API.)

class User
  attr_accessible :preferences
end

We then create a user, storing a hash within preferences (a string column in the users table):

u = User.create :preferences => { :background => "black" }

When we look up preferences, it will come back to us as YAML.

u.reload
u.preferences # => "---\n:background: black\n"

While we could run this through the YAML load function to recreate our hash, there is a nicer way. Enter serialize.

class User
  attr_accessible :preferences
  serialize :preferences
end

By calling the serialize method with whatever attributes we wish to serialize, we’re telling ActiveRecord to store and return Ruby objects, in particular arrays and hashes. Observe.

u = User.create :preferences => { :background => "black" }
u.reload
u.preferences # => { :background => "black" }

We create our object specifying a Ruby hash, store it in the database, and then upon reloading, that object returns to us as we inserted it. Beautiful. This works for “arrays, hashes, and other non-mappable objects” according to the API.

So clever tricks aside, why bother? In my mind, the serialize function is an excellent alternative to creating a new table. While we could create a preferences table and associate it with the user table through a belongs_tohas_many relationship, it certainly seems that an additional table is overkill, especially since in the example above we will only be storing a couple of key-value pairs.