Different Fields Multiplication in MongoDB - mongodb

Question

Can we multiple two different fields from different collections in mongoDB?
any help will be highly appreciated...

Answer 1

Yes, you can using the Aggregation Pipeline $multiply operator. https://docs.mongodb.com/manual/reference/operator/aggregation/multiply/
What you want to do is join two collections together using $lookup https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/. In this case, I'll join the accounts and transactions collections on the account_id field.
Then we can project the fields we want to multiply. In this case, I'm getting the first element in the account array, which represents the account document I'm joining from the accounts collection.
Finally, I can multiply the two fields together.
[{
$lookup: {
from: 'accounts',
localField: 'account_id',
foreignField: 'account_id',
as: 'account'
}
}, {
$project: {
account: {
$arrayElemAt: ["$account", 0]
},
transaction_count: "$transaction_count",
}
}, {
$project: {
product: {
$multiply: ["$transaction_count", "$account.limit"]
}
}
}]
To reproduce my solution above, create a free cluster in Atlas (https://www.mongodb.com/cloud/atlas) and then load the sample data. Navigate to the Cluster's Collections. Then navigate to the sample_analytics database and the transactions collection. Then navigate to the Aggregation tab. Here you can create an Aggregation Pipeline stage by stage. It's incredibly helpful so you can see the output of each stage as you build the next. Below is a screenshot of the Aggregation Pipeline I described in my solution above.
If you don't have experience with the Aggregation Pipeline, I highly recommend MongoDB University's free course: https://university.mongodb.com/courses/M121/about

Answer 2

MongoDB aggregation operations allows us join two collections with $lookup method and compute field operation (i.e $multiply)
Given
"collection": [
{
id: 1,
"total": 5
},
{
id: 2,
"total": 2
}
],
"collection2": [
{
collId: 1,
"total": 3
},
{
collId: 2,
"total": 4
}
]
db.collection.aggregate([
{
$lookup: {
from: "collection2",
let: {
col_id: "$id",
col_total: "$total",
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$collId",
"$$col_id"
]
}
}
},
{
$project: {
summary: {
$multiply: [
"$total",
"$$col_total"
]
}
}
}
],
as: "result"
}
},
{
$addFields: {
result: {
$let: {
vars: {
tmp: {
$arrayElemAt: [
"$result",
0
]
}
},
in: "$$tmp.summary"
}
}
}
}
])
MongoPlayground
Result
[
{
"_id": ObjectId("5a934e000102030405000000"),
"id": 1,
"result": 15,
"total": 5
},
{
"_id": ObjectId("5a934e000102030405000001"),
"id": 2,
"result": 8,
"total": 2
}
]