Railscasts - Active Model

#219

Jun 21, 2010

Active Model

In Rails 3 the non-database functionality of Active Record is extracted out into Active Model. This allows you to cleanly add validations and other features to tableless models.

Tags: models

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# models/message.rb
class Message
  include ActiveModel::Validations
  include ActiveModel::Conversion
  extend ActiveModel::Naming
  
  attr_accessor :name, :email, :content
  
  validates_presence_of :name
  validates_format_of :email, :with => /^[-a-z0-9_+\.]+\@([-a-z0-9]+\.)+[a-z0-9]{2,4}$/i
  validates_length_of :content, :maximum => 500
  
  def initialize(attributes = {})
    attributes.each do |name, value|
      send("#{name}=", value)
    end
  end
  
  def persisted?
    false
  end
end

# messages_controller.rb
def create
  @message = Message.new(params[:message])
  if @message.valid?
    # TODO send message here
    flash[:notice] = "Message sent! Thank you for conacting us."
    redirect_to root_url
  else
    render :action => 'new'
  end
end

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25 comments

1. Ryan Jun 21, 2010 at 00:42

Very cool! I'd like to see more OM's in the NoSQL world to start using this, instead of cooking up their own!

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2. Matt Powell Jun 21, 2010 at 01:03

Small caveat: ActiveModel defines before_validate and after_validate callbacks, but these aren't really usable: they're mixed up with the validations themselves, and invoked in the order they were defined. So, for example, if you declare a before_validate callback after a validates_presence_of, the before_validate will be called *after* the validation, which isn't very useful.

There's a change in edge now that moves before_validation and friends from ActiveRecord to ActiveModel to fix this, so expect that to make it to the release candidate.

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3. dhc Jun 21, 2010 at 10:06

Is it possible to just inherit from ActiveModel::Base?

If so, is there any good reason not to do this, instead of picking and choosing modules and having to dig through code to figure out which dependencies are missing?

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4. daniel Jun 21, 2010 at 18:02

same question that dhc posted.

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5. jack Jun 22, 2010 at 00:07

informative stuff

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6. Steve D Jun 22, 2010 at 10:44

Would also like response to dhc question. From some angles it looks like Rails 3 has a lot of features that come as a box of d-i-y components which you have to know how to assemble youself.

Surely this wasn't the point of Rails?

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7. jonas Jun 23, 2010 at 04:39

I think this is very dangerous...

send("#{name}=", value)

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8. Jacob Swanner Jun 23, 2010 at 23:27

instead of including ActiveModel::Conversion module, extending ActiveModel::Naming module, and defining the persisted? method...

Another option would have been to change the form_for call in the view. Just change "form_for @message" to "form_for :message, :url => messages_path"

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9. Steve D Jun 24, 2010 at 07:22

Would really like answer to dhc question, even if it makes me look even more stupid than I feel already.

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10. Casey Jun 24, 2010 at 13:49

@jonas, why? Rails does this in a similar way. Except all it does is check for protected attributes that shouldn't be mass assigned first and that the object responds to "#{key}=". If you want to add your own ability to have protected attributes, then go for it. I just don't think that's in the scope of this railscast.

http://github.com/rails/rails/blob/master/activerecord/lib/active_record/base.rb#L1532

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11. Dave Jun 24, 2010 at 14:39

Hey faithful Railscast fans:

I just wanted to point out that several of the spammers have websites that also allow comments.

I'm just sayin'...

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12. Mark Jun 25, 2010 at 03:58

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13. Mark Jun 25, 2010 at 11:22

By the way, it's missing rails 3 tag.

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14. Dan Simpson Jun 29, 2010 at 19:33

@dhc (Since people are asking): You could define an "ActiveModel::Base" class and include the functionality you want for your app. The nice thing about the modular architecture is that you can do exactly what you are looking to do, but it's not forced upon you. So just define your own base class and include the modules you want for all of your inheriting classes. Don't get this confused with ActiveRecord, as that a different set of classes and modules, that happen to leverage the ActiveModel classes/modules.

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15. Owen Jul 06, 2010 at 15:35

Very nice screencast, Ryan! I had the same problem with ActiveRecord on rails 2 which requires some hacks in order to make the model not db specific. Great work on introducing ActiveModel! You mentioned downloading the rails 3 codes and read it. I just found out a neat tool that makes it easier to read codes from a source control system on your browser, it's called CodeFaces, here is the restful link to read the rails code on GitHub: http://codefaces.org/http://github.com/rails/rails

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16. Pete Jul 08, 2010 at 07:54

I made a gem called basic_model, based on this approach:

http://github.com/petebrowne/basic_model
http://rubygems.org/gems/basic_model

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Active Model

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