Ruby 2.0 introduces the named parameters feature.
When it's used in conjuction with varargs parameters, Ruby tries to
call the #to_hash method in the arguments given when invoking the
method.

Mongoid::Criteria doesn't implement #to_hash, but it has a #method_missing
that invokes entries, which fetches data from Mongodb. When this
is performed with a huge database, it causes Mongodb to hang
for a long time.

Sample code that triggers the problem:

class HugeCollection
  include Mongoid::Document
end

def a(*args, named_parameter: nil); end

a(HugeCollection.all) # hangs forever

A simpler way of showing the issue is:

HugeCollection.all.anything_that_doesnt_exist # hangs forever