Mongo DB updateOne query calculation - mongodb

Question

I have this Schema:
const HeroSchema = new mongoose.Schema({
name: {
type: String,
required: true
},
power_start: {
type: Number,
required: true,
default: Math.floor(Math.random() * 10)
},
power_current: {
type: Number,
required: true,
default: 0
}
})
When a user clicks a button I want to randomly add 'power' between 1-10 to 'power_current'. Means, I need the final number in 'power_current' to be the sum of 'power_start' + random number + 'power_current'
'power start' getting random number as the default.
I just started to write and stuck:
updateHeroPower: async({id})=>{
const res = await HeroSchema.updateOne({
_id: id
},{
$set:{
}
})
}
Thanks

Answer 1

For Mongo version 4.2+ you can use pipelined updates like so:
const res = await HeroSchema.updateOne({
_id: id
},
[
{
$set: {
power_current: {$sum: ["$power_current", "$power_start", Math.floor(Math.random() * 10)]}
}
}
]
)
Older Mongo version do not have the power to access document fields within the update object.
This means you'll have to split it into 2 calls. First read the document and only then update it using the information you fetched.
One thing to note is I'm not sure you actually want the logic you requested as summing power_current and power_start every single time will actually inflate the score. I think what you want to do is sum random with power_start if power_current does not exist, and otherwise sum it with power_current as it already embodies power_start value after the initial sum.
Assuming my assumption is correct you can achieve this with $ifNull
const res = await HeroSchema.updateOne({
_id: id
},
[
{
$set: {
power_current: {$sum: [{$ifNull: ["$power_current", "$power_start"]}, Math.floor(Math.random() * 10)]}
}
}
]
)