something like this? (from mongo activerecord docs)

mongo_mapper_problem.rb

# multiple options; works fine
User.all(:age => [1,2,3])            
# >> [#<User age: 3>]

# using the $lt operator; works fine
User.all(:age => {'$lt' => 10})
# >> [#<User age: 3>]

# combining multiple options with the $lt operator; fails.
User.all(:age => {'$lt' => 10, '$in' => [50])             
# >> nil