Phusion Corporate Blog

We’ve just released default_value_for version 1.0.5. default_value_for is a Rails plugin for supporting default values in ActiveRecord models. Like this:

class User < ActiveRecord::Base
  default_value_for :age, 10
end

User.new.age   # => 10

You can read more about this plugin at Github.

Version 1.0.5 fixes support for Rails 3.0 and Rails 3.1. Unfortunately we had to remove one feature due to changes in ActiveRecord 3.1. It is no longer possible to access associations in the default_value_for_block. The following is no longer possible:

class User < ActiveRecord::Base
  belongs_to :organization

  default_value_for :name do |model|
    model.name = "Drone for #{model.organization.name}"
  end
end

evil_organization.name  # => "Evil Organization"
user = evil_organization.users.create
user.name               # => "Drone for Evil Organization"??

One would expect the last expression to evaluate to “Drone for Evil Organization”. On Rails 2 and Rails 3.0, it does, but on Rails 3.1 it does not. Because of the way the ‘organization’ attribute is assigned to the model by ActiveRecord, we are no longer able to support this behavior. Inside the default_value_for block, model.organization would evaluate to nil.

EncryptedCookieStore is similar to Ruby on Rails’s CookieStore (it saves session data in a cookie), but it uses encryption so that people can’t read what’s in the session data. This makes it possible to store sensitive data in the session.

EncryptedCookieStore is written for Rails 2.3. Other versions of Rails have not been tested.

Note: This is not ThinkRelevance’s EncryptedCookieStore. In the Rails 2.0 days they wrote an EncryptedCookieStore, but it seems their repository had gone defunct and their source code lost. This EncryptedCookieStore is written from scratch by Phusion.

Source code at http://github.com/FooBarWidget/encrypted_cookie_store

Installation and usage

First, install it:

./script/plugin install git://github.com/FooBarWidget/encrypted_cookie_store.git

Then edit config/initializers/session_store.rb and set your session store to EncryptedCookieStore:

ActionController::Base.session_store = EncryptedCookieStore

You need to set a few session options before EncryptedCookieStore is usable. You must set all options that CookieStore needs, plus an encryption key that EncryptedCookieStore needs. In session_store.rb:

ActionController::Base.session = {
        # CookieStore options...
        :key            => '_session',     # Name of the cookie which contains the session data.
        :secret         => 'b4589cc9...',  # A secret string used to generate the checksum for
                                           # the session data. Must be longer than 64 characters
                                           # and be completely random.

        # EncryptedCookieStore options...
        :encryption_key => 'c306779f3...', # The encryption key. See below for notes.
}

The encryption key must be a hexadecimal string of exactly 32 bytes. It should be entirely random, because otherwise it can make the encryption weak.

You can generate a new encryption key by running rake secret:encryption_key. This command will output a random encryption key that you can then copy and paste into your environment.rb.

Operational details

Upon generating cookie data, EncryptedCookieStore generates a new, random initialization vector for encrypting the session data. This initialization vector is then encrypted with 128-bit AES in ECB mode. The session data is first protected with an HMAC to prevent tampering. The session data, along with the HMAC, are then encrypted using 256-bit AES in CFB mode with the generated initialization vector. This encrypted session data + HMAC are then stored, along with the encrypted initialization vector, into the cookie.

Upon unmarshalling the cookie data, EncryptedCookieStore decrypts the encrypted initialization vector and use that to decrypt the encrypted session data + HMAC. The decrypted session data is then verified against the HMAC.

The reason why HMAC verification occurs after decryption instead of before decryption is because we want to be able to detect changes to the encryption key and changes to the HMAC secret key, as well as migrations from CookieStore. Verifying after decryption allows us to automatically invalidate such old session cookies.

EncryptedCookieStore is quite fast: it is able to marshal and unmarshal a simple session object 5000 times in 8.7 seconds on a MacBook Pro with a 2.4 Ghz Intel Core 2 Duo (in battery mode). This is about 0.174 ms per marshal+unmarshal action. See rake benchmark in the EncryptedCookieStore sources for details.

EncryptedCookieStore vs other session stores

EncryptedCookieStore inherits all the benefits of CookieStore:

• It works out of the box without the need to setup a seperate data store (e.g. database table, daemon, etc).
• It does not require any maintenance. Old, stale sessions do not need to be manually cleaned up, as is the case with PStore and ActiveRecordStore.
• Compared to MemCacheStore, EncryptedCookieStore can “hold” an infinite number of sessions at any time.
• It can be scaled across multiple servers without any additional setup.
• It is fast.
• It is more secure than CookieStore because it allows you to store sensitive data in the session.

There are of course drawbacks as well:

• It is prone to session replay attacks. These kind of attacks are explained in the Ruby on Rails Security Guide. Therefore you should never store anything along the lines of is_admin in the session.
• You can store at most a little less than 4 KB of data in the session because that’s the size limit of a cookie. “A little less” because EncryptedCookieStore also stores a small amount of bookkeeping data in the cookie.
• Although encryption makes it more secure than CookieStore, there’s still a chance that a bug in EncryptedCookieStore renders it insecure. We welcome everyone to audit this code. There’s also a chance that weaknesses in AES are found in the near future which render it insecure. If you are storing *really* sensitive information in the session, e.g. social security numbers, or plans for world domination, then you should consider using ActiveRecordStore or some other server-side store.

JRuby: Illegal Key Size error

If you get this error (and your code works with MRI)…

    Illegal key size

    [...]/vendor/plugins/encrypted_cookie_store/lib/encrypted_cookie_store.rb:62:in `marshal'

…then it probably means you don’t have the “unlimited strength” policy files installed for your JVM. Download and install them. You probably have the “strong” version if they are already there.

As a workaround, you can change the cipher type from 256-bit AES to 128-bit by
inserting the following in config/initializer/session_store.rb:

EncryptedCookieStore.data_cipher_type = 'aes-128-cfb'.freeze  # was 256

Please note that after changing to 128-bit AES, EncryptedCookieStore still requires a 32 bytes hexadecimal encryption key, although only half of the key is actually used.

Introduction

The default_value_for plugin allows one to define default values for ActiveRecord models in a declarative manner. For example:

class User < ActiveRecord::Base
  default_value_for :name, "(no name)"
  default_value_for :last_seen do
    Time.now
  end
end

u = User.new
u.name       # => "(no name)"
u.last_seen  # => Mon Sep 22 17:28:38 +0200 2008

We at Phusion use it for generating UUIDs for models.

Note: critics might be interested in the “When (not) to use default_value_for?” section. Please read on.

Installation

Install with:

./script/plugin install git://github.com/FooBarWidget/default_value_for.git

See also the AgileWebDevelopment Plugins entry.

If you like this plugin, then please consider donating and/or recommending us:

Hongli Lai

Ninh Bui

The default_value_for method

The default_value_for method is available in all ActiveRecord model classes.

The first argument is the name of the attribute for which a default value should be set. This may either be a Symbol or a String.

The default value itself may either be passed as the second argument:

default_value_for :age, 20

…or it may be passed as the return value of a block:

default_value_for :age do
  if today_is_sunday?
    20
  else
    30
  end
end

If you pass a value argument, then the default value is static and never changes. However, if you pass a block, then the default value is retrieved by calling the block. This block is called not once, but every time a new record is instantiated and default values need to be filled in.

The latter form is especially useful if your model has a UUID column. One can generate a new, random UUID for every newly instantiated record:

class User < ActiveRecord::Base
  default_value_for :uuid do
    UuidGenerator.new.generate_uuid
  end
end

User.new.uuid  # => "51d6d6846f1d1b5c9a...."
User.new.uuid  # => "ede292289e3484cb88...."

Note that record is passed to the block as an argument, in case you need it for whatever reason:

class User < ActiveRecord::Base
  default_value_for :uuid do |x|
    x   # <--- a User object
    UuidGenerator.new.generate_uuid
  end
end

Rules

Instantiation of new record

Upon instantiating a new record, the declared default values are filled into the record. You’ve already seen this in the above examples.

Retrieval of existing record

Upon retrieving an existing record, the declared default values are not filled into the record. Consider the example with the UUID:

user = User.create
user.uuid   # => "529c91b8bbd3e..."

user = User.find(user.id)
# UUID remains unchanged because it's retrieved from the database!
user.uuid   # => "529c91b8bbd3e..."

Mass-assignment

If a certain attribute is being assigned via the model constructor’s mass-assignment argument, that the default value for that attribute will not be filled in:

user = User.new(:uuid => "hello")
user.uuid   # => "hello"

However, if that attribute is protected by attr_protected or attr_accessible, then it will be filled in:

class User < ActiveRecord::Base
  default_value_for :name, 'Joe'
  attr_protected :name
end

user = User.new(:name => "Jane")
user.name   # => "Joe"

Inheritance

Inheritance works as expected. All default values are inherited by the child
class:

class User < ActiveRecord::Base
  default_value_for :name, 'Joe'
end

class SuperUser < User
end

SuperUser.new.name   # => "Joe"

Attributes that aren’t database columns

default_value_for also works with attributes that aren’t database columns. It works with anything for which there’s an assignment method:

# Suppose that your 'users' table only has a 'name' column.
class User < ActiveRecord::Base
  default_value_for :name, 'Joe'
  default_value_for :age, 20
  default_value_for :registering, true

  attr_accessor :age

  def registering=(value)
    @registering = true
  end
end

user = User.new
user.age    # => 20
user.instance_variable_get('@registering')    # => true

Caveats

A conflict can occur if your model class overrides the ‘initialize’ method, because this plugin overrides ‘initialize’ as well to do its job.

class User < ActiveRecord::Base
  def initialize  # <-- this constructor causes problems
    super(:name => 'Name cannot be changed in constructor')
  end
end

We recommend you to alias chain your initialize method in models where you use default_value_for:

class User < ActiveRecord::Base
  default_value_for :age, 20

  def initialize_with_my_app
    initialize_without_my_app(:name => 'Name cannot be changed in constructor')
  end

  alias_method_chain :initialize, :my_app
end

Also, stick with the following rules:

• There is no need to alias_method_chain your initialize method in models that don’t use default_value_for.
• Make sure that alias_method_chain is called after the last default_value_for occurance.

When (not) to use default_value_for?

You can also specify default values in the database schema. For example, you can specify a default value in a migration as follows:

create_table :users do |t|
  t.string    :username,  :null => false, :default => 'default username'
  t.integer   :age,       :null => false, :default => 20
  t.timestamp :last_seen, :null => false, :default => Time.now
end

This has the same effect as passing the default value as the second argument to default_value_for:

user = User.new
user.username   # => 'default username'
user.age        # => 20
user.timestamp  # => Mon Sep 22 18:31:47 +0200 2008

It’s recommended that you use this over default_value_for whenever possible.

However, it’s not possible to specify a schema default for serialized columns. With default_value_for, you can:

class User < ActiveRecord::Base
  serialize :color
  default_value_for :color, [255, 0, 0]
end

And if schema defaults don’t provide the flexibility that you need, then default_value_for is the perfect choice. For example, with default_value_for you could specify a per-environment default:

class User < ActiveRecord::Base
  if RAILS_ENV == "development"
    default_value_for :is_admin, true
  end
end

Or, as you’ve seen in an earlier example, you can use default_value_for to generate a default random UUID:

class User < ActiveRecord::Base
  default_value_for :uuid do
    UuidGenerator.new.generate_uuid
  end
end

Or you could use it to generate a timestamp that’s relative to the time at which the record is instantiated:

class User < ActiveRecord::Base
  default_value_for :account_expires_at do
    3.years.from_now
  end
end

User.new.account_expires_at   # => Mon Sep 22 18:43:42 +0200 2008
sleep(2)
User.new.account_expires_at   # => Mon Sep 22 18:43:44 +0200 2008

Finally, it’s also possible to specify a default via an association:

# Has columns: 'name' and 'default_price'
class SuperMarket < ActiveRecord::Base
  has_many :products
end

# Has columns: 'name' and 'price'
class Product < ActiveRecord::Base
  belongs_to :super_market

  default_value_for :price do |product|
    product.super_market.default_price
  end
end

super_market = SuperMarket.create(:name => 'Albert Zwijn', :default_price => 100)
soap = super_market.products.create(:name => 'Soap')
soap.price   # => 100

What about before_validate/before_save?

True, before_validate and before_save does what we want if we’re only interested in filling in a default before saving. However, if one wants to be able to access the default value even before saving, then be prepared to write a lot of code. Suppose that we want to be able to access a new record’s UUID, even before it’s saved. We could end up with the following code:

# In the controller
def create
  @user = User.new(params[:user])
  @user.generate_uuid
  email_report_to_admin("#{@user.username} with UUID #{@user.uuid} created.")
  @user.save!
end

# Model
class User < ActiveRecord::Base
  before_save :generate_uuid_if_necessary

  def generate_uuid
    self.uuid = ...
  end

  private
    def generate_uuid_if_necessary
      if uuid.blank?
        generate_uuid
      end
    end
end

The need to manually call generate_uuid here is ugly, and one can easily forget to do that. Can we do better? Let’s see:

# Controller
def create
  @user = User.new(params[:user])
  email_report_to_admin("#{@user.username} with UUID #{@user.uuid} created.")
  @user.save!
end

# Model
class User < ActiveRecord::Base
  before_save :generate_uuid_if_necessary

  def uuid
    value = read_attribute('uuid')
    if !value
      value = generate_uuid
      write_attribute('uuid', value)
    end
    value
  end

  # We need to override this too, otherwise User.new.attributes won't return
  # a default UUID value. I've never tested with User.create() so maybe we
  # need to override even more things.
  def attributes
    uuid
    super
  end

  private
    def generate_uuid_if_necessary
      uuid  # Reader method automatically generates UUID if it doesn't exist
    end
end

That’s an awful lot of code. Using default_value_for is easier, don’t you think?

What about other plugins?

I’ve only been able to find 2 similar plugins:

• Default Value: http://agilewebdevelopment.com/plugins/default_value
• ActiveRecord Defaults: http://agilewebdevelopment.com/plugins/activerecord_defaults

Default Value appears to be unmaintained; its SVN link is broken. This leaves only ActiveRecord Defaults. However, it is semantically dubious, which leaves it wide open for corner cases. For example, it is not clearly specified what ActiveRecord Defaults will do when attributes are protected by attr_protected or attr_accessible. It is also not clearly specified what one is supposed to do if one needs a custom initialize method in the model.

I’ve taken my time to thoroughly document default_value_for’s behavior.

Credits

I’ve wanted such functionality for a while now and it baffled me that ActiveRecord doesn’t provide a clean way for me to specify default values. After reading http://groups.google.com/group/rubyonrails-core/browse_thread/thread/b509a2fe2b62ac5/3e8243fa1954a935, it became clear that someone needs to write a plugin. This is the result.

Thanks to Pratik Naik for providing the initial code snippet on which this plugin is based on: http://m.onkey.org/2007/7/24/how-to-set-default-values-in-your-model

If you like this plugin, then please consider donating and/or recommending us:

Hongli Lai

Ninh Bui