How to set default values in Rails?

"Correct" is a dangerous word in Ruby. There's usually more than one way to do anything. If you know you'll always want that default value for that column on that table, setting them in a DB migration file is the easiest way:

class SetDefault < ActiveRecord::Migration
  def self.up
    change_column :people, :last_name, :type, :default => "Doe"
  end

  def self.down
    # You can't currently remove default values in Rails
    raise ActiveRecord::IrreversibleMigration, "Can't remove the default"
  end
end

Because ActiveRecord autodiscovers your table and column properties, this will cause the same default to be set in any model using it in any standard Rails app.

However, if you only want default values set in specific cases -- say, it's an inherited model that shares a table with some others -- then another elegant way is do it directly in your Rails code when the model object is created:

class GenericPerson < Person
  def initialize(attributes=nil)
    attr_with_defaults = {:last_name => "Doe"}.merge(attributes)
    super(attr_with_defaults)
  end
end

Then, when you do a GenericPerson.new(), it'll always trickle the "Doe" attribute up to Person.new() unless you override it with something else.

If you are referring to ActiveRecord objects, you have (more than) two ways of doing this:

1. Use a :default parameter in the DB

E.G.

class AddSsl < ActiveRecord::Migration
  def self.up
    add_column :accounts, :ssl_enabled, :boolean, :default => true
  end

  def self.down
    remove_column :accounts, :ssl_enabled
  end
end

More info here: http://api.rubyonrails.org/classes/ActiveRecord/Migration.html

2. Use a callback

E.G. before_validation_on_create

More info here: http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html#M002147

In case you're dealing with a Model, you can use the Attriutes API in Rails 5+ http://api.rubyonrails.org/classes/ActiveRecord/Attributes/ClassMethods.html#method-i-attribute

just add a migration with a proper column name and then in the model set it with:

class StoreListing < ActiveRecord::Base
  attribute :country, :string, default: 'PT'
end

Generate a migration and use change_column_default, is succinct and reversible:

class SetDefaultAgeInPeople < ActiveRecord::Migration[5.2]
  def change
    change_column_default :people, :age, { from: nil, to: 0 }
  end
end

If you are just setting defaults for certain attributes of a database backed model I'd consider using sql default column values - can you clarify what types of defaults you are using?

There are a number of approaches to handle it, this plugin looks like an interesting option.

The suggestion to override new/initialize is probably incomplete. Rails will (frequently) call allocate for ActiveRecord objects, and calls to allocate won't result in calls to initialize.

If you're talking about ActiveRecord objects, take a look at overriding after_initialize.

These blog posts (not mine) are useful:

Default values Default constructors not called

[Edit: SFEley points out that Rails actually does look at the default in the database when it instantiates a new object in memory - I hadn't realized that.]

You can override the constructor for the ActiveRecord model.

Like this:

def initialize(*args)
  super(*args)
  self.attribute_that_needs_default_value ||= default_value
  self.attribute_that_needs_another_default_value ||= another_default_value
  #ad nauseum
end