Rails: How can I set default values in ActiveRecord?

How can I set default value in ActiveRecord?

I see a post from Pratik that describes an ugly, complicated chunk of code: http://m.onkey.org/2007/7/24/how-to-set-default-values-in-your-model

class Item < ActiveRecord::Base  
  def initialize_with_defaults(attrs = nil, &block)
    initialize_without_defaults(attrs) do
      setter = lambda { |key, value| self.send("#{key.to_s}=", value) unless
        !attrs.nil? && attrs.keys.map(&:to_s).include?(key.to_s) }
      setter.call('scheduler_type', 'hotseat')
      yield self if block_given?
    end
  end
  alias_method_chain :initialize, :defaults
end

I have seen the following examples googling around:

  def initialize 
    super
    self.status = ACTIVE unless self.status
  end

and

  def after_initialize 
    return unless new_record?
    self.status = ACTIVE
  end

I've also seen people put it in their migration, but I'd rather see it defined in the model code.

Is there a canonical way to set default value for fields in ActiveRecord model?

There are several issues with each of the available methods, but I believe that defining an after_initialize callback is the way to go for the following reasons:

1. default_scope will initialize values for new models, but then that will become the scope on which you find the model. If you just want to initialize some numbers to 0 then this is not what you want.
2. Defining defaults in your migration also works part of the time... As has already been mentioned this will not work when you just call Model.new.
3. Overriding initialize can work, but don't forget to call super!
4. Using a plugin like phusion's is getting a bit ridiculous. This is ruby, do we really need a plugin just to initialize some default values?
5. Overriding after_initialize is deprecated as of Rails 3. When I override after_initialize in rails 3.0.3 I get the following warning in the console:

DEPRECATION WARNING: Base#after_initialize has been deprecated, please use Base.after_initialize :method instead. (called from /Users/me/myapp/app/models/my_model:15)

Therefore I'd say write an after_initialize callback, which lets you default attributes in addition to letting you set defaults on associations like so:

  class Person < ActiveRecord::Base
    has_one :address
    after_initialize :init

    def init
      self.number  ||= 0.0           #will set the default value only if it's nil
      self.address ||= build_address #let's you set a default association
    end
  end    

Now you have just one place to look for initialization of your models. I'm using this method until someone comes up with a better one.

Caveats:

1. For boolean fields do:

   self.bool_field = true if self.bool_field.nil?

   See Paul Russell's comment on this answer for more details

2. If you're only selecting a subset of columns for a model (ie; using select in a query like Person.select(:firstname, :lastname).all) you will get a MissingAttributeError if your init method accesses a column that hasn't been included in the select clause. You can guard against this case like so:

   self.number ||= 0.0 if self.has_attribute? :number

   and for a boolean column...

   self.bool_field = true if (self.has_attribute? :bool_value) && self.bool_field.nil?

   Also note that the syntax is different prior to Rails 3.2 (see Cliff Darling's comment below)

Rails 5+

You can use the attribute method within your models, eg.:

class Account < ApplicationRecord
  attribute :locale, :string, default: 'en'
end

You can also pass a lambda to the default parameter. Example:

attribute :uuid, :string, default: -> { SecureRandom.uuid }

The second argument is the type and it can also be a custom type class instance, for example:

attribute :uuid, UuidType.new, default: -> { SecureRandom.uuid }

We put the default values in the database through migrations (by specifying the :default option on each column definition) and let Active Record use these values to set the default for each attribute.

IMHO, this approach is aligned with the principles of AR : convention over configuration, DRY, the table definition drives the model, not the other way around.

Note that the defaults are still in the application (Ruby) code, though not in the model but in the migration(s).

Some simple cases can be handled by defining a default in the database schema but that doesn't handle a number of trickier cases including calculated values and keys of other models. For these cases I do this:

after_initialize :defaults

def defaults
   unless persisted?
    self.extras||={}
    self.other_stuff||="This stuff"
    self.assoc = [OtherModel.find_by_name('special')]
  end
end

I've decided to use the after_initialize but I don't want it to be applied to objects that are found only those new or created. I think it is almost shocking that an after_new callback isn't provided for this obvious use case but I've made do by confirming whether the object is already persisted indicating that it isn't new.

Having seen Brad Murray's answer this is even cleaner if the condition is moved to callback request:

after_initialize :defaults, unless: :persisted?
              # ":if => :new_record?" is equivalent in this context

def defaults
  self.extras||={}
  self.other_stuff||="This stuff"
  self.assoc = [OtherModel.find_by_name('special')]
end

The after_initialize callback pattern can be improved by simply doing the following

after_initialize :some_method_goes_here, :if => :new_record?

This has a non-trivial benefit if your init code needs to deal with associations, as the following code triggers a subtle n+1 if you read the initial record without including the associated.

class Account

  has_one :config
  after_initialize :init_config

  def init_config
    self.config ||= build_config
  end

end