MongoDB: Update field using value from another field

Question

Using Mongo shell, I am trying to add a field which has the same value as an existing field for all documents in a collection. Assuming we have two documents :

{
   'foo': 'str1'
}

and

{
   'foo': 'str2'
}

I would like to insert a new field 'foo_new' which has the respective value of 'foo' as its value, so that the documents become

{
   'foo': 'str1'
   'foo_new': 'str1'
}

and

{
   'foo': 'str2'
   'foo_new': 'str2'
}

The command I use to update the collection ('coll' say) in Mongo shell is

db.coll.update({}, {$set: {'foo_new': '$foo'}}, {multi: true})

The result of running this command are the two updated documents

{
   'foo': 'str1'
   'foo_new': '$foo'
}

and

{
   'foo': 'str2'
   'foo_new': '$foo'
}

i.e. '$foo' is being interpreted as a literal for some reason.

Does this answer your question? how to update a number field using another number field in MongoDB — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Feb 28, 2020 at 15:36
@whoami: In theory it does. It's exactly what I tried, but my Mongo shell would interpret the "$a" as the string '$a' not as 2, the value of the field whose key is 'a'. — John S.
– John S., Commented Feb 28, 2020 at 17:01
S : There is a difference in what you've tried with what has to be done, .update() should be in aggregation pipeline :: [ ] not as an object !! What is your MongoDB version ? — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Feb 28, 2020 at 17:02
@whoami: Thanks for pointing out! I'll take the read & write back approach, but thanks for offering to provide a query. — John S.
– John S., Commented Feb 28, 2020 at 17:12

Adarsh Madrecha · Accepted Answer · 2022-01-10 15:12:45Z

22

Try this snippet:

db.<collection>.update({}, [{$set: {'foo_new': '$foo'}}], {"multi": true})

Note the [] square brackets in 2nd argument. More info: https://stackoverflow.com/a/37280419/4050261

edited Jan 10, 2022 at 15:12

Adarsh Madrecha

8,20514 gold badges78 silver badges129 bronze badges

answered Feb 28, 2020 at 17:04

Basu_C

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5 Comments

whoami - fakeFaceTrueSoul Over a year ago

stackoverflow.com/questions/60418883/…

pranav Over a year ago

If we have create a new document, can we have the new field auto populated with this value. Is there a way to achieve this? I will basically have a calculation based on 2 other fields. Will the calculation reflect for the new field if we add a new document?

Kane Over a year ago

This isn't a valid solution for Mongo 4.0.

Kyle H Over a year ago

FWIW I ran into this same issue and didn't need to add the {"multi": true} portion, just adding the square brackets around my set criteria did the trick.

Arkin Solomon Over a year ago

Don't use update() with {"multi": true} just use updateMany() or updateOne()

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