Maximum distance with Mongoid's Model.near method
Place.near(:coordinates => location.reverse)
# must reverse the resulting coordinates array because mongo stores them backwards [lng,lat]

db.places.find( { loc : { $near : [50,50] , $maxDistance : 5 } } )

Place.where(:coordinates => {"$near" => location.reverse , '$maxDistance' => 20.fdiv(69)})