Rails 3 after_initialize not running when create is called with a block

Question

I am trying to set some default values for an object using after_initialize. The problem I am having is that I would like this to be called no matter how the object is created.

My class:

class Foo < ActiveRecord::Base

  serialize :data

  after_initialize :init

  def init
    self.data ||= {}
    self.bar ||= "bar"
    self.baz ||= "baz"
  end

end

Everything works fine if I call Foo.new, Foo.new(:bar => "things") and Foo.create(:baz => 'stuff'). However when I use a block with create the after_initialize callback doesn't get run.

obj = Foo.create do |f|
  f.bar = "words"
  f.data = { :attr_1 => 1, :attr_2 => 2 }
end

This just yields obj.baz => nil instead of "baz" with the other attributes set correctly.

Am I missing something with the way callbacks are executed, with the differences with calling create with a block and without or are default values getting clobbered by the block?

UPDATE

Found the issue.

It turns out that calling create with block and without are subtly different. When you call create without a block and just pass in a hash of parameters, for all intents and purposes you are calling Foo.new({<hash of argument>}).save, and the after_initialize callback gets executed right before the save like you would expect.

When you call create with a block something a little different happens. The order of events is Foo.new is called with whatever arguments you pass in, then after_initialize gets called, then the block gets run. So if you are using the block (as I was) interchangeably with the hash parameters just to make things a little bit more readable you could get bit because your after_initialize is run before all of the parameters you intend to set are actually set.

I got bit because I was doing some extra work in the after_initialize setting some extra required attributes based on the value of what was getting passed. Since nothing was actually set when after_initialize got called, nothing got set correctly and my validations failed.

I ended up having to make to calls to init. Once on after_initialize and once on before_validation. Not the cleanest, but it solved the issue.

Thanks go to Brandon for pointing me in the right direction.

If you do the same thing but with Foo.new (i.e. with the block method of declaration), does the same thing happen? — Andrew Marshall
– Andrew Marshall, Commented Mar 5, 2011 at 3:55
@HMCFletch - great explanation of what's happening - thanks for sharing — Peter Nixey
– Peter Nixey, Commented May 6, 2014 at 9:45

Michelle Tilley · Accepted Answer · 2011-03-05 04:25:27Z

7

I'm unable to reproduce this. I happen to have an application handy with the following (simplified) class:

class Service < ActiveRecord::Base
  serialize        :data, Hash
  after_initialize :create_default_data
  attr_accessible  :data, :token

  protected

    def create_default_data
      self.data ||= Hash.new
    end
end

Here's an IRB session:

ruby-1.9.2-p136 :001 > obj = Service.create do |s|
ruby-1.9.2-p136 :002 >     s.token = "abc"
ruby-1.9.2-p136 :003?>   end
 => #<Service id: 22, user_id: nil, type: nil, data: {}, created_at: "2011-03-05 04:18:00", updated_at: "2011-03-05 04:18:00", token: "abc"> 
ruby-1.9.2-p136 :004 > obj.data
 => {}

As you can see, data as initialized in the after_initialize method to an empty hash. The Rails code indicates this makes sense as well; in create:

def create(attributes = nil, &block)
  if attributes.is_a?(Array)
    attributes.collect { |attr| create(attr, &block) }
  else
    object = new(attributes)
    yield(object) if block_given?
    object.save
    object
  end
end

So create calls new and assigns the value to object before it yields. Here's the relevant part in new:

def initialize(attributes = nil)
  # truncated for space
  result = yield self if block_given?
  run_callbacks :initialize
  result
end

As you can see, new unconditionally calls the initialize callbacks before it returns, and thus before create even yields to the block you pass. By the time your block gets the object, the after_initialize method has already executed.

Double check that (1) your version of Rails is up to date (3.0.5 as of now I believe) and that (2) nothing is setting baz without you realizing it.

answered Mar 5, 2011 at 4:25

Michelle Tilley

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2 Comments

HMCFletch Over a year ago

Thanks for the piece in bold. That was exactly what was causing my problems. I wrote up a little explanation at the bottom of my question detailing what was happening. Thanks, for the help.

Michelle Tilley Over a year ago

Very interesting, and great that you nailed down what was going on. I can't imagine you are the only person to ever be bitten by this...

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