Ruby-on-rails – Rails 3 + activerecord, the best way to “mass update” a single field for all the records that meet a condition

To get a copy, use the clone (or dup for rails 3.1+) method:

# rails < 3.1
new_record = old_record.clone

#rails >= 3.1
new_record = old_record.dup

Then you can change whichever fields you want.

ActiveRecord overrides the built-in Object#clone to give you a new (not saved to the DB) record with an unassigned ID.
Note that it does not copy associations, so you'll have to do this manually if you need to.

Rails 3.1 clone is a shallow copy, use dup instead...

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)