block the creation of multiple object of a class

Question

How do I limit the object of any class to one. My class looks like :

class Speaker
  include Mongoid::Document
  field :name, :type => String
end

I just want to let a single instance of speaker . One way would be to add a validation which would check the number of objects already present of the Speaker class. Is there a ruby way of doing thing ?

If you want only one speaker, does it makes sense to store it in db. I would have extracted it to a config file. — rubish, Aug 19 '11 at 19:49
@rubish I need to allow the admin to change the value. How would I do that efficiently if I don't save it in the database ? — Prabesh Shrestha, Aug 20 '11 at 03:03
@Slawosz I think that is the best thing I can do right now. Just waiting if I can find any better solution. — Prabesh Shrestha, Aug 22 '11 at 11:34

score 10 · Answer 1 · answered Aug 19 '11 at 11:34

10

How about using the Singleton module?

answered Aug 19 '11 at 11:34

Frank Schmitt

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how do I change the value . I also want the value to persist. – Prabesh Shrestha Aug 19 '11 at 17:24
If it's the Admin USER changing the value, then this answer has a code smell. Otherwise see http://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons for lengthly debate. – Alec Wenzowski Aug 31 '11 at 22:42

score 8 · Answer 2 · answered Aug 19 '11 at 11:36

8

In this case I would write proper validation:

validate :only_one

def only_one
   errors.add(:base, "Only one Speaker can exist") if self.count > 0 
end

answered Aug 19 '11 at 11:36

Sławosz

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This will fail if the record already exists and you're updating it. – Steven Soroka Aug 26 '11 at 06:14
Good point, but you can write: validate :only_one, :on => :create – Sławosz Aug 26 '11 at 08:13
2

``NoMethodError: undefined method `count` for #`` - `self` in the context of a validation method is an instance, so `self.count` needs to be changed to `Speaker.count` or `self.class.count` – Abe Voelker Apr 02 '13 at 22:17

score 3 · Answer 3 · answered Aug 24 '11 at 21:41

I recommend using a class/module that is tailored to storing configuration values rather rolling your own on top of a vanilla ActiveRecord model.

I use a old copy of the rails-settings plugin with some custom modification (it still works just fine in Rails 3). There are also a number of variant offerings listed on Github, so feel free to look and take your pick.

score 3 · Answer 4 · answered Aug 26 '11 at 06:15

3

Why not provide a default Speaker object, and just not provide controller actions for create or delete?

Seems the simplest solution by far.

answered Aug 26 '11 at 06:15

Steven Soroka

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score 3 · Answer 5 · edited May 23 '17 at 11:55

I see you're using Mongoid

The functionality you request is not available using mongoid validations.

Therefore, you will need to write your own. before_validation is a supported callback and chained Speaker.all.count methods are available to your model.

class Speaker
  include Mongoid::Document
  field :name, :type => String
  before_validation(:ensure_has_only_one_record, :on => :create)
  def ensure_has_only_one_record
    self.errors.add :base, "There can only be one Speaker." if Speaker.all.count > 0
  end
end

However, the best practice is to put all key/value settings in a single table.

andersonvom · Answer 6 · 2013-04-08T12:34:40.983

Using the Singleton module and a little overriding of its methods, I believe this works and it's thread safe (on ruby 1.8):

class Speaker 

  include Singleton
  include Mongoid::Document
  field :name, :type => String

  @@singleton__instance__ = nil
  @@singleton__mutex__ = Mutex.new

  def self.instance
    return @@singleton__instance__ if @@singleton__instance__
    @@singleton__mutex__.synchronize {
      return @@singleton__instance__ if @@singleton__instance__
      @@singleton__instance__ = self.first
      @@singleton__instance__ ||= new()
    }
    @@singleton__instance__
  end

  def destroy
    @@singleton__mutex__.synchronize {
      super
      @@singleton__instance__ = nil
    }
  end

end

Thanks for pointing that out. Back in Ruby 1.8 [that was not the case](http://www.ruby-forum.com/topic/4412037). — andersonvom, Apr 08 '13 at 12:44

block the creation of multiple object of a class

6 Answers6

I see you're using Mongoid

However, the best practice is to put all key/value settings in a single table.