StoreField - Nested fields for ActiveRecord::Store

Rails 3.2 introduced ActiveRecord::Store, which offers simple single-column key-value stores.

It's a nice feature, but its accessors are limited to primitive values (e.g. String, Integer, etc.) and it doesn't work out of the box if you want to store structured values. (e.g. Hash, Set, etc.)

Here's an example.

class User < ActiveRecord::Base
  store :options, accessors: [ :tutorials, :preference ]
end

user = User.new
user.tutorials[:quick_start] = :visited     # => NoMethodError: undefined method `[]=' for nil:NilClass

There are two ways to solve this problem - a. break down options into multiple columns like tutorials and preference, or b. define an accessor method for each to initialize with an empty Hash when accessed for the first time.

The former is bad because the TEXT (or BLOB) column type could be stored off-page when it gets big and you could hit some strange bugs and/or performance penalty. Furthermore, adding columns kills the primary purpose of having key-value store - you use this feature because you don't like migrations, right? So it's two-fold bad.

StoreField takes the latter approach. It defines accessors that initialize with an empty Hash or Set automatically. Now you have a single TEXT column for everything!

Changelog:

v2.0.0: Hash-type supports keys option to add accessors for validation. Set-type with values option now adds a validation rather than raising an exception.

Usage

Add this line to your application's Gemfile.

gem 'store_field'

Define store_field in a model class, following the store method.

class User < ActiveRecord::Base
  store :storage
  store_field :tutorials
end

Now the previous example works perfectly.

user = User.new
user.tutorials[:quick_start] = :finished

When no option is given, it defaults to the first serialized column, using Hash-type. So store_field :tutorials is equivalent to the following.

store_field :tutorials, in: :storage, type: Hash

Hash-type features

When the keys option is given for Hash-type, convenience accessors are automatically defined, which can be used for validation.

class User < ActiveRecord::Base
  store_field :tutorials, keys: [ :quick_start ]

  validates :tutorials_quick_start, inclusion: { in: [ :started, :finished ], allow_nil: true }
end

user = User.new
user.tutorials_quick_start = :started
user.valid?
 => true

Set-type features

In addition to Hash-type, StoreField supports Set-type. To use Set-type, simply pass type: Set option.

It turns out that Set-type is extremely useful most of the time when you think what you need is Array.

store_field :funnel, type: Set

It defines several utility methods - set_[field], unset_[field], set_[field]? and unset_[field]?.

cart = Cart.new
cart.funnel                     # => #<Set: {}>
cart.set_funnel(:add_item)
cart.set_funnel(:checkout)
cart.set_funnel?(:checkout)     # => true
cart.funnel                     # => #<Set: {:add_item, :checkout}>

set_[field] and unset_[field] return self, so you can call save in chain.

cart.set_funnel(:checkout).save!    # => true

Also you can enumerate acceptable values for validation.

class Cart < ActiveRecord::Base
  store_field :funnel, type: Set, values: [ :add_item, :checkout ]
end

cart = Cart.new
cart.set_funnel(:bogus)
cart.valid?
 => false

Use cases for the Set-type

Set-type is a great way to store an arbitrary number of named states.

Consider you have a system that sends an alert when some criteria have been met.

if user.bandwidth_usage > 250.megabytes
  Email.to user, message: 'Your data plan usage is nearing 300MB limit'
end

Depending on at what time the above code gets run (daily, hourly, etc.), email could be sent multiple times. To prevent duplicate alerts, you need to store the state in the database when one is successfully delivered.

class User < ActiveRecord::Base
  store :storage
  store_field :delivered, type: Set
end

if user.bandwidth_usage > 250.megabytes and !user.set_delivered?(:nearing_limit)
  Email.to user, message: 'Your data plan usage is nearing 300MB limit'
  user.set_delivered(:nearing_limit).save
end

That way, the user won't receive the same alert again, until unset_delivered is called when the next billing cycle starts.

YAML serialization

ActiveRecord::Store uses YAML to serialize Ruby objects. A StoreField will be stored as follows:

---
:funnel: !ruby/object:Set
  hash:
    :add_item: true
    :checkout: true

As you can see, the Set class internally uses Hash for its storage.

There is a known compatibility problem between psych and syck, be sure to use psych from the beginning.

YAML::ENGINE.yamler     # => "psych"