mongodb aggregate division between to arrays last elements - mongodb

Question

I have a feed with products that I store in a db (so Im not in control over the structure of the data or its properties). The feed is stored in a mongoDB and accessible later on a website. This is example of the data stored in db.
{
productName: 'ABC',
price: {
previous: [null, null, 100],
latest: [200, 200, 200],
},
};
On the site I want to list products in order of largest discount. I.e price.latest / price.previous. I've tried alot and I want to do something like this:
$set: {
discount: {
$cond: {
if: {
$and: [
{ $gt: [{ $last: '$price.latest' }, null] },
{ $gt: [{ $last: '$price.previous' }, null] },
],
},
then: {
$divide: [
{ $last: '$price.latest' },
{ $last: '$price.previous' },
],
},
else: null,
},
},
}
I cant get it to work and I dont know if its the null check of division crashes. :/

Answer 1

Your syntax is correct as can be seen here, the only edge case you left out is the case that price.previous is equal to 0. This was an easy fix as you just have to change the null to 0 in your $gt conditions:
{
$set: {
discount: {
$cond: {
if: {
$and: [
{
$gt: [
{
$last: "$price.latest"
},
0
]
},
{
$gt: [
{
$last: "$price.previous"
},
0
]
}
]
},
then: {
$divide: [
{
$last: "$price.latest"
},
{
$last: "$price.previous"
}
]
},
else: null
}
}
}
}
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