MongoDB decrement until zero - mongodb

Question

I would like to achieve an operation in MongoDB that would be analogous to doc.value = max(doc.value - amount, 0). I could do it by fetching document, updating its value then saving it, but is it possible with an atomic operation to avoid problems with synchronisation of parallel decrements?

Answer 1

It is, in fact, possible to achieve with a single operation. All you need is an aggregation pipeline inside an update operator.
Let's say you have a doc that looks like this:
{
"key": 1,
value: 30
}
You want to subtract x from value and if the resulting value is less than zero, set value to 0, otherwise set it to whatever value - x is. Here is an update aggregator you need. In this example I am subtracting 20 from value.
db.collection.update({
key: 1
},
[
{
$set: {
"value": {
$cond: [
{
$gt: [
{
$subtract: [
"$value",
20
]
},
0
]
},
{
$subtract: [
"$value",
20
]
},
0
]
}
}
}
])
The result will be:
{
"key": 1,
"value": 10
}
But if you change 20 to, say 44, the result is:
{
"key": 1,
"value": 0
}
Here is a Playground for you: https://mongoplayground.net/p/Y9yO6v9Oca8

Answer 2

Kudos to codemonkey's response for providing a solution using aggregation pipelines for an atomic transaction.
Here's a slightly simpler aggregation pipeline that takes advantage of the $max operator:
db.collection.update({},
[
{
$set: {
"value": {
$max: [
0,
{
$subtract: [
"$value",
20
]
}
]
}
}
}
],
{
multi: true
})
The pipeline sets the value to the maximum of 0 and the result of the decrement.
Playground