Constant Cache

When your database has tables that store lookup data, there is a tendency to provide those values as constants in the model. If you have an account_statuses table with a corresponding model, your constants may look like this:

class AccountStatus
  ACTIVE   = 1
  PENDING  = 2
  DISABLED = 3
end

There are a couple of problems with this approach:

As you add more lookup data to the table, you need to ensure that you're updating your models along with the data.

The constants are stored as integer values and need to match up exactly with the data that's in the table (not necessarily a bad thing), but this solution forces you to write code like this:

Account.new(:username => 'preagan', :status => AccountStatus.find(AccountStatus::PENDING))

This requires multiple calls to find and obfuscates the code a bit. Since classes in Ruby are executable code, we can cache the objects from the database at load time and use them in your application.

Changes

Major changes in version 0.2.0:

Split versions for DM and AR. For dm use: gem 'dm-constant-cache', '0.2.0', :require => 'constant_cache'

Major changes in version 0.1.0:

In order to support DataMapper as well as ActiveRecord, and to reduce dependencies in the gem itself we've decided to make ConstantCache a module that must be included in the class you wish to call cache_constants on. If you wish to have all AR classes include the module, simply add an initializer to do ActiveRecord::Base.send(:include, ConstantCache)

The cache now call #all instead of #find(:all) because it is supported by both ORMs.

The only other major change in the is a change in the method name #caches_constants to #cache_constants (cache is a verb, implying an action or event)

Installation

This code is packaged as a gem, so simply use the gem command to install:

gem install constant_cache

Example

"Out of the box," the constant_cache gem assumes that you want to use the 'name' column to generate constants from a column called 'name' in your database table. Assuming this schema:

create_table :account_statuses do |t|
  t.string :name, :description
end

AccountStatus.create!(:name => 'Active',   :description => 'Active user account')
AccountStatus.create!(:name => 'Pending',  :description => 'Pending user account')
AccountStatus.create!(:name => 'Disabled', :description => 'Disabled user account')

We can use the plugin to cache the data in the table:

class AccountStatus # uses ActiveRecord or DataMapper
  include ConstantCache
  
  cache_constants
end

Now you can write code that's a little cleaner and not use multiple unnecessary find calls:

Account.new(:username => 'preagan', :status => AccountStatus::PENDING)

If the column you want to use as the constant isn't 'name', you can set that in the model. If we have :name, :slug, and :description, we can use 'slug' instead:

class AccountStatus # uses ActiveRecord or DataMapper
  include ConstantCache
  
  cache_constants :key => :slug
end

The value for the constant is truncated at 64 characters by default, but you can adjust this as well:

class AccountStatus # uses ActiveRecord or DataMapper
  include ConstantCache
  
  cache_constants :limit => 16
end

Acknowlegements

Thanks to Dave Thomas for inspiring me to write this during his Metaprogramming talk at a Rails Edge conference in early 2007.

Copyright (c) 2009 Patrick Reagan and Tony Pitale of Viget Labs, released under the MIT license