How to create view to read from two collections in mongoDB? - mongodb

Question

Started with mongoDB syntax and use in project.
I am looking for a solution where I can combine more than two collections with couple of condition to create a view.
Here is my collection Range
/* 1 */
{
"_id" : ObjectId("1"),
"range" : {
"start" : "00"
},
"products" : [
{
"id" : "01",
"name" : "FirstProduct",
"type" : "First Type"
},
{
"id" : "02",
"name" : "Second Product",
"type" : "Second Type"
},
{
"id" : "03",
"name" : "Third Product",
"type" : "Third Type"
},
]
}
/* 2 */
{
"_id" : ObjectId("2"),
"range" : {
"start" : "100",
},
"products" : [
{
"id" : "01",
"name" : "First Product",
"type" : "First Type"
},
{
"id" : "02",
"name" : "Second Product",
"type" : "Second Type"
}
]
}
/* 3 */
{
"_id" : ObjectId("3"),
"range" : {
"start" : "500",
},
"products" : [
{
"id" : "01",
"name" : "First Product",
"type" : "First Type"
},
{
"id" : "02",
"name" : "Second Product",
"type" : "Second Type"
}
]
}
Second Collection. Stock
/* 1 */
{
"_id" : ObjectId("1"),
"range" : {
"start" : "00"
},
"products" : [
{
"id" : "01",
"expired" : false,
"returned" : false
},
{
"id" : "02",
"expired" : false,
"returned" : false
}
]
}
/* 2 */
{
"_id" : ObjectId("02"),
"range" : {
"start" : "100"
},
"products" : [
{
"id" : "01",
"expired" : true,
"returned" : true
},
{
"id" : "02",
"expired" : true,
"returned" : true
}
{
"id" : "03",
"expired" : true,
"returned" : true
}
]
}
Now want to have a view with combine result from above two collection above.
For each range document in Range collections
if Range.range.start = Stock.range.start
if Range.products.id = Stock.products.id
copy "expired" and "returned" field from Stock for that product and
add to Range.product
end if
end if
Return Range
So final result will something like below.
/* 1 */
{
"_id" : ObjectId("1"),
"range" : {
"start" : "00"
},
"products" : [
{
"id" : "01",
"name" : "FirstProduct",
"type" : "First Type"
"expired" : false,
"returned" : false
},
{
"id" : "02",
"name" : "Second Product",
"type" : "Second Type"
"expired" : false,
"returned" : false
}
]
}
/* 2 */
{
"_id" : ObjectId("2"),
"range" : {
"start" : "100",
},
"products" : [
{
"id" : "01",
"name" : "First Product",
"type" : "First Type",
"expired" : true,
"returned" : true
},
{
"id" : "02",
"name" : "Second Product",
"type" : "Second Type",
"expired" : true,
"returned" : true
}
]
}
/* 3 */
{
"_id" : ObjectId("3"),
"range" : {
"start" : "500",
},
"products" : [
{
"id" : "01",
"name" : "First Product",
"type" : "First Type"
},
{
"id" : "02",
"name" : "Second Product",
"type" : "Second Type"
}
]
}
I started with aggregate pipeline stages with fail to get right queries.
if anyone can help with right syntax and proper aggregate function.
Thanks in advance.

Answer 1

You need $lookup to merge the data from both collections but then you have to use $unwind to be able to match corresponding documents by product.id. In the last step you can use $group to get back an array:
db.Range.aggregate([
{
$lookup: {
from: "Stock",
localField: "range.start",
foreignField: "range.start",
as: "stock"
}
},
{
$unwind: "$stock"
},
{
$unwind: "$products"
},
{
$unwind: "$stock.products"
},
{
$match: { $expr: { $eq: [ "$products.id", "$stock.products.id" ] } }
},
{
$group: {
_id: "$_id",
"range": { $first: "$range" },
products: {
$push: {
id: "$products.id",
name: "$products.name",
type: "$products.type",
expired: "$stock.products.expired",
returned: "$stock.products.returned"
}
}
}
}
])
EDIT: Alternative solution which operates directly on arrays using $map and $filter below. The drawback is that the code is less readable but the good part is that it should return documents when there's no match and you should get better performance using this approach
db.Range.aggregate([
{
$lookup: {
from: "Stock",
localField: "range.start",
foreignField: "range.start",
as: "stock"
}
},
{
$unwind: "$stock"
},
{
$addFields: {
products: {
$map: {
input: "$products",
as: "p",
in: {
$let: {
vars: {
stockItem: {
$arrayElemAt: [
{ $filter: { input: "$stock.products", cond: { $eq: [ "$$p.id", "$$this.id" ] } } }, 0
]
}
},
in: {
$cond: [
{ $eq: [ "$$stockItem", undefined ] },
"$$p",
{
id: "$$p.id",
name: "$$p.name",
type: "$$p.type",
expired: "$$stockItem.expired",
returned: "$$stockItem.returned",
}
]
}
}
}
}
}
}
},
{
$project: {
stock: 0
}
}
])