Database
db={"orders":[{_id:"5ecf3445365eca3e58ff57c0",type:"city",name:"Toronto",CSD:"3520005",CSDTYPE:"C",PR:"35",PRNAME:"Ontario",geometry:{},country:"CAN",updatedAt:"2021-04-23T18:25:50.774+00:00",province:"ON"}],"inventory":[{_id:"5ecbe871d8ab4ab6845c5142",geometry:{},name:"henry12",user:"5cbdd019b9d9170007d15990",__v:0}]}
Query
db.orders.aggregate([{$unionWith:"inventory"},{$project:{_id:1,name:1,province:{$cond:{if:"$province",then:"$province",else:null}},type:{$cond:{if:"$type",then:"$type",else:"custom"}}}},{"$sort":{name:1}}])