Mongoid appears to implement all comparison operators as methods on symbols except eq:
(byebug) MyModel.any_of(:field.ne => 'x') #<Mongoid::Criteria selector: {"field"=>{"$ne"=>"x"}} options: {} class: MyModel embedded: false> (byebug) MyModel.any_of(:field.eq => 'x') *** NoMethodError Exception: undefined method `eq' for :field:Symbol