I have this two collections namely Inward and Outward, Both collections have similar embeded sub documents contains product, batch and quantity fields - mongodb

Question

Inward collections
{"ord" : 1,
"products" : [
{
"name" : "apple",
"qty" : "10",
"batch" : "jun-2021"
},
{
"name" : "banana",
"qty" : 20,
"batch" : "jan-2021"
}
]
}
outward collections
{
"_id" : ObjectId("5edde5487957d9efea972a74"),
"inv" : 1,
"products" : [
{
"name" : "apple",
"qty" : 13,
"batch" : "jun-2021"
}
]
}
Now, I would like to perform actual stock quantity check for particular product and batch (grouping together) both the collections

Answer 1

You may try this way:
We join them with inward.ord = outward.inv condition.
Flatten products field.
Group by product's name and batch to sum qty value.
db.inward.aggregate([
{
$lookup: {
from: "outward",
let: {
ord: "$ord",
products: "$products"
},
pipeline: [
{
$match: {
$expr: {
$eq: [ "$$ord", "$inv" ]
}
}
},
{
$project: {
products: {
$concatArrays: [
"$$products",
"$products"
]
}
}
},
{
$unwind: "$products"
},
{
$replaceWith: "$products"
}
],
as: "products"
}
},
{
$unwind: "$products"
},
{
$group: {
_id: {
batch: "$products.batch",
name: "$products.name"
},
qty: {
$sum: "$products.qty"
}
}
}
])
MongoPlayground
Note: You need to have MongoDB v4.2