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Track maximum value in array

Problem

Your document contains an array of numbers and you want to add an attribute to the document which contains the maximum value in the array. You want to ensure that the document is updated safely and atomically so that this value always represents the maximum value after any number of additions to the array.

Assumptions

• You are updating the document by its _id or another unique field.
• You know the document already exists (i.e. you are not "upserting.")

Solution

MongoDB's atomic updates to not allow you to perform in-document comparisons when updating--that is, there is no operator which will update a value if and only if it is greater than the existing value. Such an operator would render this recipe trivial.

However, you can accomplish this task with two invocations of the findAndModify command:

1. Issue a findAndModify that sets the max_value and pushes to the array at the same time. This operation only succeeds if the max_value is less than or equal to the new value.

2. If the previous operation fails, it can only be because max_value is already greater than the new value, so it is safe to push the new value without regard for max_value.

To obtain the result of the findAndModify command, take the first result that succeeds and assign it to the result variable. Because the second findAndModify only runs if the preceding operations made no updates, then we know that there can only ever be a single value of result.

The code for this operation resembles:

var result1 = null, result2 = null;

result1 = db.collection.findAndModify({
    query: {_id: ObjectId(...), max_value: {$lte: new_value}},
    update: {$push: {array: new_value}, $set: {max_value: new_value}}});

if (result1 === null ) {
    result3 = db.collection.findAndModify({
        query: {_id: ObjectId(...)},
        update: {$push: {array: new_value}}});
}

var result = result1 || result2;

Variations

If you want the result variable to include the changes made by whichever of the two findAndModifys succeeded, add new: true to the arguments to findAndModify.

If you want the array attribute of the document to contain a set of unique values, rather than an array of all values pushed, use the $addToSet operator rather than $push.