In MongoDB, method to keep the previous value of a field in a different field while updating an object?

Question

Say I have an object with field state, I want to update this field, while keeping the previous value of state in previous_state field. First, I have tried to make an update with unset-rename-set:

collection.update(query, {$unset: {previous_state: ""}, $rename: {state: "previous_state"}, $set: {state: value}})

no surprise it did not work. After reading:

I am nearly convinced that I do not have a solution to perform this in a single query. So the question is what is the best practice to do it?

Can muliple users access the database? Or doe you have only one connection. — Vincent Beltman
– Vincent Beltman, Commented Dec 17, 2014 at 14:19
I have a single writer and multiple reader users for this collection. Does it make any difference? — kokeksibir
– kokeksibir, Commented Dec 17, 2014 at 14:23
That depends on how you achieve the value of the newstate. But since you only have one writer, the answer of Lombric would be the best. — Vincent Beltman
– Vincent Beltman, Commented Dec 17, 2014 at 14:26

marcor92 · Accepted Answer · 2022-04-28 13:15:09Z

1

There are various ways to do it, depending on the version of MongoDB, and they are described in this answer from another thread: https://stackoverflow.com/a/37280419/5538923 . For MongoDB 3.4+, for example, there is this query that can be put in MongoSH:

db.collection.aggregate(
[
    { "$addFields": { 
        "previous_state": { "$concat": [ "$state" ] },
        "state": { "$concat": [ "$state", " customly modified" ] } 
    }},
    { "$out": "collection" }
])

Also note that this query works only when the MongoDB instance is not sharded. When sharded (e.g., often the case in Microsoft Azure CosmosDB), the method described in that answer for MongoDB 3.2+ works, or alternatively put a new collection as destination (in the $out close), and then import the data in the original collection, after removing all the data there.

answered Apr 28, 2022 at 13:15

marcor92

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Comments

Lombric · Accepted Answer · 2014-12-17 14:29:32Z

0

One solution (if you've got onlty one writer) could be to trigger your update in two steps:

> var previousvalue = collection.findOne(query).state;
> collection.update(query, {$set: {"previous_state": previousvalue, "state": newstatevalue}});

edited Dec 17, 2014 at 14:29

answered Dec 17, 2014 at 14:22

Lombric

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2 Comments

Inanc Gumus Over a year ago

This is not atomic and will lead to concurrency issues when more than one caller is involved (it's not scalable).

gtzinos Over a year ago

Any update on this? I would agree with @InancGumus

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