How to update a value and increment another in single-query using mongodb

Question

Is there a way to achieve the following:-

MyDocument {
    String docId;
    Integer bal;
    Set<String> actions;
}

1. I need to update the field actions with some value of a user action, say from

   [a1, a2]
   

   to

   [a1, a2, a3]
   

2. At the same time, this added action can increment the bal by some value so I need to ensure that $inc operation is also implemented for the bal field.

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One way of achieving this I could think of was incrementing the bal using one update query :

Bson searchFilter = Filters.in("docId", <someId>);

// couldn't find a cleaner way of increment in mongo-java
BasicDBObject modifiedObject = new BasicDBObject();
modifiedObject.put("$inc", new BasicDBObject().append("bal",2)); // any value to increment

mongoCollection.updateOne(searchFilter, modifiedObject);

I can thereafter execute another update document query for set of actions update.

---

Another way could possibly have been updating the documents with both the fields. For which :

• I would have to find the document first, read the current bal, perform the compute at application.
• Then update the document with both the computed fields.

---

In both the above approaches, I couldn't see a way of avoiding to query the DB twice. Is there any other workaround possible to achieve this currently?

Also, I am concerned about the atomicity with the above combination of [UPDATE + UPDATE] or [GET + UPDATE], given multiple such concurrent queries can occur, is the concern justified even?

Do let me know any further detail required.