How to return an empty ActiveRecord relation?

265
votes

If I have a scope with a lambda and it takes an argument, depending on the value of the argument, I might know that there will not be any matches, but I still want to return a relation, not an empty array:

scope :for_users, lambda { |users| users.any? ? where("user_id IN (?)", users.map(&:id).join(',')) : [] }

What I really want is a "none" method, the opposite of "all", that returns a relation that can still be chained, but results in the query being short-circuited.

9
ruby-on-railsactiverecordrelation
If you just let the query, run it will return a relation: User.where('id in (?)', []).class => ActiveRecord::Relation. Are you trying to avoid the query altogether? - Brian Deterling
Correct. If I know there can't possibly be any matches, ideally, the query could be avoided altogether. I simply added this to ActiveRecord::Base: "def self.none; where(:id => 0); end" Seems to work just fine for what I need. - dzajic
> Are you trying to avoid the query altogether? would totally make sense, kind of lame we need to hit DB for that - dolzenko

9 Answers

518
votes

There is a now a "correct" mechanism in Rails 4:

>> Model.none 
=> #<ActiveRecord::Relation []>
77
votes

A more portable solution that doesn't require an "id" column and doesn't assume there won't be a row with an id of 0:

scope :none, where("1 = 0")

I'm still looking for a more "correct" way.

45
votes

Coming in Rails 4

In Rails 4, a chainable ActiveRecord::NullRelation will be returned from calls like Post.none.

Neither it, nor chained methods, will generate queries to the database.

According to the comments:

The returned ActiveRecord::NullRelation inherits from Relation and implements the Null Object pattern. It is an object with defined null behavior and always returns an empty array of records without quering the database.

See the source code.

42
votes

You can add a scope called "none":

scope :none, where(:id => nil).where("id IS NOT ?", nil)

That will give you an empty ActiveRecord::Relation

You could also add it to ActiveRecord::Base in an initializer (if you want):

class ActiveRecord::Base
 def self.none
   where(arel_table[:id].eq(nil).and(arel_table[:id].not_eq(nil)))
 end
end

Plenty of ways to get something like this, but certainly not the best thing to keep in a code base. I have used the scope :none when refactoring and finding that I need to guarantee an empty ActiveRecord::Relation for a short time.

26
votes
scope :none, limit(0)

Is a dangerous solution because your scope might be chained upon.

User.none.first

will return the first user. It's safer to use

scope :none, where('1 = 0')
14
votes

I think I prefer the way this looks to the other options:

scope :none, limit(0)

Leading to something like this:

scope :users, lambda { |ids| ids.present? ? where("user_id IN (?)", ids) : limit(0) }
3
votes

Use scoped:

scope :for_users, lambda { |users| users.any? ? where("user_id IN (?)", users.map(&:id).join(',')) : scoped }

But, you can also simplify your code with:

scope :for_users, lambda { |users| where(:user_id => users.map(&:id)) if users.any? }

If you want an empty result, use this (remove the if condition):

scope :for_users, lambda { |users| where(:user_id => users.map(&:id)) }
2
votes

There are also variants, but all of these are making request to db

where('false')
where('null')
2
votes

It is possible and so that's:

scope :for_users, lambda { |users| users.any? ? where("user_id IN (?)", users.map(&:id).join(',')) : User.none }

http://apidock.com/rails/v4.0.2/ActiveRecord/QueryMethods/none

Correct me if I'm wrong.