Empty Scope with Ruby on Rails

Question

Following Problem:
I need something like an empty scope. Which means that this scope is emtpy, but responds to all methods a scope usually responds to. I'm currently using a little dirty hack. I simply supply "1=0" as conditions. I find this realy ugly, since it hits the database. Simply returning an empty array won't work, since the result must respond to the scoped methods.

Is there a better existing solution for this or will I need to code this myself?

Maybe some example code could help explain what i need:

class User < ActiveRecord::Base
  named_scope :admins, :conditions => {:admin => true }
  named_scope :none_dirty, :conditions => "1=0" # this scope is always empty

  def none_broken
    []
  end

  def self.sum_score # okay, a bit simple, but a method like this should work!
    total = 0
    self.all.each do |user|
      total += user.score
    end
    return total
  end
end
User.admin.sum_score # the score i want to know
User.none_drity.sum_score # works, but hits the db
User.none_broken.sum_score # ...error, since it doesn't respond to sum_score

Hemianopsia

Why would the score of an empty set ever be anything other than 0? What are you trying to accomplish here? – Provencher

You are looking for an empty collection. – Myles

@jdl: it wouldn't be something other than 0. – Hemianopsia

Answer 1

The thing you are looking for does not exist. You could implement something like this by monky patching the find method. Yet, this would be an overkill, so I recomend keeping this unless it's performance critical.

Hemianopsia

Answer 2

Rails 4 introduces the none scope.

It is to be used in instances where you have a method which returns a relation, but there is a condition in which you do not want the database to be queried.

If you want a scope to return an unaltered scope use all:

No longer will a call to Model.all execute a query immediately and return an array of records. In Rails 4, calls to Model.all is equivalent to now deprecated Model.scoped. This means that more relations can be chained to Model.all and the result will be lazily evaluated.

Aswarm

Answer 3

User.where('false')

returns an ActiveRecord::Relation with zero elements, that is a chain-able scope that won't hit the database until you actually try to access one of its elements. This is similar to PhilT's solution with ('1=0') but a little more elegant.

Melan

Answer 4

Sorry User.scoped is not what you want. As commented this returns everything. Should have paid more attention to the question.

I've seen where('1 = 0') suggested before and Rails should probably cache it as well.

Also, where('1 = 0') won't hit the database until you do .all, .each, or one of the calculations methods.

Nitrification

User.scoped returns all users, so it's not really an "empty" scope. I think he means an empty scope that would return no users. Sort of equivalent to [] but a scope. – Gothic

Answer 5

I thing you need User.scoped({})

Judijudicable

Wouldn't User.scoped({}) be equivalent to User.all? – Myles

@Swanand: they are not equivalent, scoped returns a lazy ActiveRecord::NamedScope::Scope while User.all returns an array. – Melbourne

Answer 6

How about User.where(id: nil) ?

Or User.where(_id: nil) for mongoid.

Gothic

That will still hit the database and it's not reliable (It's possible to have a record with a null id field especially legacy, non Rails databases). – Nitrification

Answer 7

Looking at your example code indicates you may not know about aggregated queries in SQL which are exposed as calculations methods in Rails:

User.sum(:score) will give you the sum of all users' scores

Take a look at Rails Guides for more info:

http://guides.rubyonrails.org/active_record_querying.html#sum

Nitrification

Ehm, no, the sum was just an example as stated above – Hemianopsia