MongoDb Aggregate nested documents with $add - mongodb

Question

I need to get sum value from nested documents.
DB document:
{
"_id": 123,
"products": [
{
"productId": 1,
"charges": [
{
"type": "che",
"amount": 100
}
]
}
]
}
i wanted to get sum value.
sumValue = products.charges.amount+20; where "products.productId" is 1 and "products.charges.type" is "che"
i tried below query but no hope:
db.getCollection('test').aggregate(
[
{"$match":{$and:[{"products.productId": 14117426}, {"products.charges.type":"che"}]},
{ $project: { "_id":0, total: { $add: [ "$products.charges.price", 20 ] } }}
]
)
please help me to solve this.

Answer 1

You have to take a look at $unwind operator which deconstructs an array to output a document for each element of array. Also take a look at add and project operators.
I assume your db query should look like this:
db.test.aggregate([
{$unwind: '$products'}, // Unwind products array
{$match: {'products.productId' : 3}}, // Matching product id
{$unwind: '$products.charges'}, // Unwind charges
{$match: {'products.charges.type' : 'che'}}, // Matching charge type of che
{$project: {'with20': {$add: ["$products.charges.amount", 20]}}}, // project total field which is value + 20
{$group: {_id : null, amount: { $sum: '$with20' }}} // total sum
])

Answer 2

You can run $reduce twice to convert your arrays into scalar value. The outer condition could be applied as $filter, the inner one can be run as $cond:
db.collection.aggregate([
{
"$project": {
_id: 0,
total: {
$reduce: {
input: { $filter: { input: "$products", cond: [ "$$this.productId", 1 ] } },
initialValue: 20,
in: {
$add: [
"$$value",
{
$reduce: {
input: "$$this.charges",
initialValue: 0,
in: {
$cond: [ { $eq: [ "$$this.type", "che" ] }, "$$this.amount", 0 ]
}
}
}
]
}
}
}
}
}
])
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