[{"_id":{"$oid":"615839a18ffb730a922af032"},"id":"1"},{"_id":{"$oid":"615839a18ffb730a922af033"},"id":"2"},{"_id":{"$oid":"615839a18ffb730a922af039"},"id":"15"},{"_id":{"$oid":"615839a18ffb730a922af040"},"id":"45"}]

db.collection.find({$expr:{$lt:[{$toInt:"$id"},5]}})