how to update a number field using another number field in MongoDB

Question

Can MongoDB allow updating a number field using another number field in same document ? Let say I have a document like below :

{
   "_id" : "1",
   "a" : 2,
   "b" : 3
}

a and b fields are number fields(decimal,int...) my goal is update field b using field a How can I set b like b = 2*a. Is this possible ?

On 3.6 you don't have an option to direct update, You need to first read & then update !! For v4.2 you can do it in one call !! Anyway 3.6 is so old better upgrade your version asap.. — whoami - fakeFaceTrueSoul
– whoami - fakeFaceTrueSoul, Commented Feb 26, 2020 at 17:12
@whoami i tried your solution and it works in verison 4.2, thanks for your help — Faruk Yazici
– Faruk Yazici, Commented Feb 27, 2020 at 6:47

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-02-27 06:51:57Z

5

Note :

On v3.6 you don't have an option for direct update, You need to first read & process in code, then update.
For v>= 4.2 you can do it in one call as .update() will accept aggregation pipeline & things can be done in one update call to DB. You can try this :

Query :

db.getCollection('collectionName').update({}, [{ $set: { b: { $multiply: ["$a", 2] } } }])

answered Feb 27, 2020 at 6:51

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