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StoreField - Nested fields for ActiveRecord::Store

tag: v0.0.1

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README.md

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 initializes with an empty Hash or Set automatically. Now you have a single TEXT column for everything!

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 the Hash datatype. So store_field :tutorials is equivalent to the following.

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

Typing support for Set

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

It turns out that Set 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

Use cases for the Set type

Set is a great way to store an arbitrary number of 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.

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