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.

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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 .

your initialize method in models that don’t use . 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 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: