db={"orders":[{"_id":1},{"_id":2,"the_field":201},{"_id":3,"the_field":[301,302,303]}],"author":[{_id:"steevej",at:"gmail",dot:"com",for:"mongodb forum 267813"}]}

db.orders.aggregate([{$set:{"the_field":{$switch:{branches:[{case:{"$eq":["missing",{"$type":"$the_field"}]},then:[]},{case:{"$eq":["array",{"$type":"$the_field"}]},then:"$the_field"}],default:["$the_field"]}}}}])