How to access the fields from arrays of a object in two different collections? - mongodb

Question

This is locations collection data.
{
_id: "1",
location: "loc1",
sublocations: [
{
_id: 2,
sublocation: "subloc1",
},
{
_id: 3,
sublocation: "subloc2",
}
]
},
{
_id: "4",
location: "loc2",
sublocations: [
{
_id: 5,
sublocation: "subloc1",
},
{
_id: 6,
sublocation: "subloc2",
}
]
}
This is products collection data
{
_id: "1",
product: "product1",
prices: [
{
_id: 2,
sublocationid: 2, //ObjectId of object in sublocations array
price: 500
},
{
_id: 3,
sublocationid: 5, //ObjectId of object in sublocations array
price: 200
}
]
}
Now I need to get the sublocation in product schema in the prices array. Expected result is as below.
{
_id: "1",
product: "product1",
prices: [
{
_id: 2,
sublocationid: 3,
sublocation: "subloc2",
price: 500
},
{
_id: 3,
sublocationid: 5,
sublocation: "subloc1"
price: 200
}
]
}
To achieve it, I did it like in the following way.
First, performing aggregation on locations collection - $unwind the sublocations array and store the $out in the new collection.
Second, perform aggregation on 'products' collection - $unwind the prices, $lookup the sublocationid from the new collection and $group them.
Third, after getting data delete the data of new collection.
Is there any other simplified way? Please let me know if there is any.

Answer 1

If you want to stick with 3.4 version, you can try this query:
db.products.aggregate([
{
$unwind: {
"path": "$prices"
}
},
{
$lookup: {
"from": "locations",
"localField": "prices.sublocationid",
"foreignField": "sublocations._id",
"as": "locations"
}
},
{
$unwind: {
"path": "$locations"
}
},
{
$unwind: {
"path": "$locations.sublocations"
}
},
{
$addFields: {
"keep": {
"$eq": [
"$prices.sublocationid",
"$locations.sublocations._id"
]
}
}
},
{
$match: {
"keep": true
}
},
{
$addFields: {
"price": {
"_id": "$prices._id",
"sublocationid": "$prices.sublocationid",
"sublocation": "$locations.sublocations.sublocation",
"price": "$prices.price"
}
}
},
{
$group: {
"_id": "$_id",
"product": { "$first": "$product" },
"prices": { "$addToSet": "$price" }
}
}
]);
It's not as nice as 3.6 version though, because of a higher memory consumption.

Answer 2

You can try below aggregation query in 3.6 version.
Since both local field and foreign field are array you have to $unwind both to do equality comparison.
For this you will have to use new $lookup syntax.
$match with $expr provides comparsion between document fields to look up the location's sublocation document for each product's sublocation id.
$project to project the matching sublocation doc.
$addFields with $arrayElemAt to convert the looked up sublocation array into a document.
$group to push all prices with matching sublocation's document for each product.
db.products.aggregate[
{
"$unwind": "$prices"
},
{
"$lookup": {
"from": "locations",
"let": {
"prices": "$prices"
},
"pipeline": [
{
"$unwind": "$sublocations"
},
{
"$match": {
"$expr": [
"$$prices.sublocationid",
"$sublocations._id"
]
}
},
{
"$project": {
"sublocations": 1,
"_id": 0
}
}
],
"as": "prices.sublocations"
}
},
{
"$addFields": {
"prices.sublocations": {
"$arrayElemAt": [
"$prices.sublocations",
0
]
}
}
},
{
"$group": {
"_id": "$_id",
"product": {
"$first": "$product"
},
"prices": {
"$push": "$prices"
}
}
}
])