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Provides a way to specify default values for ActiveRecord models

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README.rdoc

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

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

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

The default_values method

As a shortcut, you can use default_values to set multiple default values at once.

default_values :age  => 20
               :uuid => lambda { UuidGenerator.new.generate_uuid }

The difference is purely aesthetic. If you have lots of default values which are constants or constructed with one-line blocks, default_values may look nicer. If you have default values constructed by longer blocks, default_value_for suit you better. Feel free to mix and match.

As a side note, due to specifics of Ruby's parser, you cannot say,

default_value :uuid { UuidGenerator.new.generate_uuid }

because it will not parse. This is in part the inspiration for the default_values syntax.

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

Default values are not duplicated

The given default values are not duplicated when they are filled in, so if you mutate a value that was filled in with a default value, then it will affect all subsequent default values:

class Author < ActiveRecord::Base
  # This model only has a 'name' attribute.
end

class Book < ActiveRecord::Base
  belongs_to :author

  # By default, a Book belongs to a new, unsaved author.
  default_value_for :author, Author.new
end

book1 = Book.new
book1.author.name  # => nil
# This mutates the default value:
book1.author.name = "John"

book2 = Book.new
book2.author.name  # => "John"

You can prevent this from happening by passing a block to default_value_for, which returns a new object instance every time:

class Book < ActiveRecord::Base
  belongs_to :author

  default_value_for :author { Author.new }
end

book1 = Book.new
book1.author.name  # => nil
book1.author.name = "John"

book2 = Book.new
book2.author.name  # => nil

The main reason why default values are not duplicated is because not all objects can be duplicated. For example, Fixnum responds to dup, but calling dup on a Fixnum will raise an exception.

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' 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 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: m.onkey.org/2007/7/24/how-to-set-default-values-in-your-model

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