How can I set default values in ActiveRecord?

Question

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?

Answer 1

If the column happens to be a 'status' type column, and your model lends itself to the use of state machines, consider using the aasm gem, after which you can simply do

  aasm column: "status" do
    state :available, initial: true
    state :used
    # transitions
  end

It still doesn't initialize the value for unsaved records, but it's a bit cleaner than rolling your own with init or whatever, and you reap the other benefits of aasm such as scopes for all your statuses.

Answer 2

use default_scope in rails 3

api doc

ActiveRecord obscures the difference between defaulting defined in the database (schema) and defaulting done in the application (model). During initialization, it parses the database schema and notes any default values specified there. Later, when creating objects, it assigns those schema-specified default values without touching the database.

discussion

Answer 3

From the api docs http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html Use the before_validation method in your model, it gives you the options of creating specific initialisation for create and update calls e.g. in this example (again code taken from the api docs example) the number field is initialised for a credit card. You can easily adapt this to set whatever values you want

class CreditCard < ActiveRecord::Base
  # Strip everything but digits, so the user can specify "555 234 34" or
  # "5552-3434" or both will mean "55523434"
  before_validation(:on => :create) do
    self.number = number.gsub(%r[^0-9]/, "") if attribute_present?("number")
  end
end

class Subscription < ActiveRecord::Base
  before_create :record_signup

  private
    def record_signup
      self.signed_up_on = Date.today
    end
end

class Firm < ActiveRecord::Base
  # Destroys the associated clients and people when the firm is destroyed
  before_destroy { |record| Person.destroy_all "firm_id = #{record.id}"   }
  before_destroy { |record| Client.destroy_all "client_of = #{record.id}" }
end

Surprised that his has not been suggested here

Answer 4

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)

Answer 5

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).

Answer 6

Similar questions, but all have slightly different context: - How do I create a default value for attributes in Rails activerecord's model?

Best Answer: Depends on What You Want!

If you want every object to start with a value: use after_initialize :init

You want the new.html form to have a default value upon opening the page? use https://stackoverflow.com/a/5127684/1536309

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 

If you want every object to have a value calculated from user input: use before_save :default_values You want user to enter X and then Y = X+'foo'? use:

class Task < ActiveRecord::Base
  before_save :default_values
  def default_values
    self.status ||= 'P'
  end
end