Rails migration set current date as default value

You can set default date on migration from Rails 5

create_table :posts do |t|
  t.datetime :published_at, default: -> { 'NOW()' }
end

Here a link from rails repo

You can pass a lambda for dynamic initializers.

create_table "test", :force => true do |t|
  t.date  "day", default: -> { 'CURRENT_DATE' }
end

Old answer

Rails does not support dynamic default values in migrations. Whatever is in your migration during its execution will be set at the DB level and will stay that way until the migration is rolled back, overridden, or reset. But you can easily add dynamic defaults at the model level since it's evaluated at runtime.

1) Setting default values using after_initialize callback

class Test
  def after_initialize
    self.day ||= Date.today if new_record?
  end
end

Use this approach only if you need to access the attribute after initialization and before saving the record. This approach has extra processing cost while loading a query result, as the block has to be executed for every result object.

2) Setting default values using before_create callback

class Test
  before_create do
    self.day = Date.today unless self.day
  end
end

This callback is triggered by a create call on your model. There are many more callbacks. For example, setting the date before validation on create and update.

class Test
  before_validation on: [:create, :update] do
    self.day = Date.today
  end
end

3) Using the default_value_for gem

class Test
  default_value_for :day do
    Date.today
  end
end