MongoDB - Array element as variable

Question

Lets say here is the same document:

{
"_id" : 1.0,
"custname" : "metlife",
"address" : {
    "city" : "Bangalore",
    "country" : "INDIA"
 }
}

And if I want to push an extra field to this document, something like below:

db.customers.updateMany(
  {"address.country":"INDIA"},
  {$push : {city: "$address.country"}}

)

It results in wrong update:

{
"_id" : 1.0,
"custname" : "metlife",
"address" : {
    "city" : "Bangalore",
    "country" : "INDIA"
 },
 "city" : "$address.city"
}

Instead of this:

{
"_id" : 1.0,
"custname" : "metlife",
"address" : {
    "city" : "Bangalore",
    "country" : "INDIA"
 },
 "city" : "Bangalore"
}

How do I achieve the above result?

mickl · Accepted Answer · 2018-03-08T17:19:09.160

1

You can't refer to other field values in update currently (more here). There is a workaround in aggregation framework (using $out) but it will replace entire collection.

I think that you can consider using $rename in your case. It will not add new field but it can move city to the top level of your document.

db.customers.updateMany({"address.country":"INDIA"}, {$rename: {"address.city": "city"}})

will give you following structure:

{ "_id" : 1, "custname" : "metlife", "address" : { "country" : "INDIA" }, "city" : "Bangalore" }

edited Mar 08 '18 at 17:19

answered Mar 08 '18 at 17:01

mickl

48,568
9
60
89

Thanks for the note :) – Rahul Raj Mar 08 '18 at 17:09
Can you tell me what are those `false` and `true` stands for? Does it represent `upsert/multi` ? – Rahul Raj Mar 08 '18 at 17:14
@RahulRaj sorry, I tested this case on update operation, you don't need them for `updateMany` – mickl Mar 08 '18 at 17:19

Taki · Answer 2 · 2018-03-08T17:13:36.567

0

like @mickl said : You can't refer to other field values in update currently, you have to iterate through your collection to update the documents, try this :

db.eval(function() { 
    db.collection.find({"address.country":"INDIA"}).forEach(function(e) {
        e.city= e.address.city;
        db.collection.save(e);
    });
});

Keep in mind that this will block the DB until all updates are done.

edited Mar 08 '18 at 17:13

answered Mar 08 '18 at 17:06

Taki

17,320
4
26
47

You meant to say DB will be locked until this execution is completed? – Rahul Raj Mar 08 '18 at 17:08

score -1 · Answer 3 · answered Mar 08 '18 at 17:02

-1

try this

db.customers.updateMany(
  {"address.country":"INDIA"},
  {$push : {city: "address.country"}}

)

remove $ sign

answered Mar 08 '18 at 17:02

Gowtham Sooryaraj

3,639
3
13
22

MongoDB - Array element as variable

3 Answers3