Prevent creating records on ActiveRecord null relations

In some cases, you want an object that behaves like an ActiveRecord relation but does not contain any objects. In Rails 4, you can call none on a relation and it returns an empty relation:

current_user.comments.none
# => #<ActiveRecord::AssociationRelation []>

This is an implementation of the null object design pattern. It behaves like a relation, but it never queries the database.

Imagine a site where only registered users can leave comments, but every page lists your most recent comments in the sidebar. We could of course check in the view whether or not a user is logged in, but in this case we’ll implement a Guest class which represents a non-registered user of the site. A registered user has_many :comments, so we need to simulate this on our Guest:

class Guest
  def comments
    Comment.none
  end
end

Cool, so now we can pretend we’re dealing with a regular user in our views! current_user returns our guest if the user is not logged in:

current_user.comments
# => #<ActiveRecord::AssociationRelation []>

Without having to worry about guest users in the view ever again, the website ran happily ever after. That is, until the server logs started reporting errors:

PG::NotNullViolation: ERROR: null value in column “user_id” violates not-null constraint

The error seems to originate from a piece of controller code where comments are being created:

current_user.comments.create(comment_params)

As it turns out, our null relation doesn’t contain any objects, but we can still use it to create new records!

This happens because of the way the null relation works. When diving into the Rails code, NullRelation turns out to be a module which defines several methods like count, update_all, and pluck which always return empty values.

The none method of a relation looks like this:

def none
  extending(NullRelation)
end

The extending method extends only this instance with the given modules. This overrides all methods in the relation which are also defined in NullRelation. But since create hasn’t been defined in NullRelation, it can still be used.

We can also make use of the extending method to create a true null relation in our Guest class. It also takes a block, which we will use to define a create method:

class Guest
  def comments
    Comment.none.extending do
      def create(*args)
        new(*args).tap do |record|
          record.errors.add(:base, 'Guests cannot add comments')
        end
      end

      def create!(*args)
        raise ActiveRecord::RecordInvalid, create(*args)
      end
    end
  end
end

Now when a guest attempts to add a new comment, a comment object is indeed initialized, but not persisted. In addition, we follow the ActiveModel behaviour and add a validation error:

current_user.comments.create.errors.full_messages
# => ["Guests cannot add comments"]

The controller and views won’t need to handle the guest user any different now for adding comments, since this approach follows the Rails way quite nicely.