= 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

=== Rails 3

Add it to your Gemfile:

  gem "default_value_for"

=== Rails 2

default_value_for no longer supports Rails 2! The last version that supported Rails
2 is version 1.0.7. Install that with:

  ./script/plugin install git://github.com/FooBarWidget/default_value_for.git -r release-1.0.7


== 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
  
== default_value_for options

* allows_nil (default: true) - Sets explicitly passed nil values if option is set to true.
 
You can pass this options hash as 2nd parameter and have to pass the default value through the :value option in this case e.g.:

  default_value_for :age, :value => 20, :allows_nil => false 
 
You can still pass the default value through a block:
  
  default_value_for :uuid, :allows_nil => false do
    UuidGenerator.new.generate_uuid
  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 }

If you like to override default_value_for options for each attribute you can do so:

  default_values :age  => { :value => 20 },
                 :uuid => { :value => lambda { UuidGenerator.new.generate_uuid }, :allows_nil => false }

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_for :uuid { UuidGenerator.new.generate_uuid }

because it will not parse. One needs to write

  default_value_for(:uuid) { UuidGenerator.new.generate_uuid }

instead. 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 in the following case, 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..."

But when the declared default value is set to not allow nil and nil is passed the default values will be set on retrieval.
Consider this example:

  default_value_for(:number, :allows_nil => false) { 123 }

  user = User.create
  
  # manual SQL by-passing active record and the default value for gem logic through ActiveRecord's after_initialize callback
  user.update_attribute(:number, nil)
  
  # declared default value should be set
  User.find(user.id).number # => 123 # = declared default value

=== 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"
  
  # the without protection option will work as expected
  user = User.new({:name => "Jane"}, :without_protection => true)
  user.name   # => "Jane"

Explicitly set nil values for accessible attributes will be accepted:

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

  user = User(:name => nil)
  user.name # => nil

... unless the accessible attribute is set to not allowing nil:

  class User < ActiveRecord::Base
    default_value_for :name, 'Joe', :allows_nil => false
  end

  user = User(:name => nil)
  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 duplicated

The given default values are duplicated when they are filled in, so if
you mutate a value that was filled in with a default value, then it will
not 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 does not mutate the default value:
  book1.author.name = "John"
  
  book2 = Book.new
  book2.author.name  # => nil

However the duplication is shallow. If you modify any objects that are
referenced by the default value then it will affect subsequent default values:

  class Author < ActiveRecord::Base
    attr_accessor :useless_hash
    default_value_for :useless_hash, { :foo => [] }
  end
  
  author1 = Author.new
  author1.useless_hash    # => { :foo => [] }
  # This mutates the referred array:
  author1.useless_hash[:foo] << 1
  
  author2 = Author.new
  author2.useless_hash    # => { :foo => [1] }

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

  class Author < ActiveRecord::Base
    attr_accessor :useless_hash
    default_value_for :useless_hash do
      { :foo => [] }
    end
  end
  
  author1 = Author.new
  author1.useless_hash    # => { :foo => [] }
  author1.useless_hash[:foo] << 1
  
  author2 = Author.new
  author2.useless_hash    # => { :foo => [] }

=== 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.

If your default value is accidentally similar to default_value_for's options hash wrap your default value like this:

  default_value_for :attribute_name, :value => { :value => 123, :other_value => 1234 }

== 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
  end

This has similar effects as passing the default value as the second argument to
+default_value_for+:

  default_value_for(:username, 'default_username')
  default_value_for(:age, 20)

Default values are filled in whether you use the schema defaults or the
default_value_for defaults:

  user = User.new
  user.username   # => 'default username'
  user.age        # => 20

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