I have a simple class:

class Article
  include Mongoid::Document
end

Let's create at least 102 records (because the default batchSize of mongodb is 101):

102.times { Article.create }

Now if I make this query with the QueryCache enabled:

Mongoid::QueryCache.cache { Article.all.any?; Article.all.map(&:id).size }
=> 101

It does not return the correct number of articles which should be 102.

After some investigation, the reason is that the 2 commands make the same queries... except that the map makes a getMore to get the rest of the list. The QueryCache prevents the getMore from being made.

A workaround could be to use exists? or empty? but we clearly should not have an issue with any? too.