Problems aggregating MongoDB - mongodb

Question

I am having problems aggregating my Product Document in MongoDB.
My Product Document is:
{
"_id" : ObjectId("5d81171c2c69f45ef459e0af"),
"type" : "T-Shirt",
"name" : "Panda",
"description" : "Panda's are cool.",
"image" : ObjectId("5d81171c2c69f45ef459e0ad"),
"created_at" : ISODate("2019-09-17T18:25:48.026+01:00"),
"is_featured" : false,
"sizes" : [
"XS",
"S",
"M",
"L",
"XL"
],
"tags" : [ ],
"pricing" : {
"price" : 26,
"sale_price" : 8
},
"categories" : [
ObjectId("5d81171b2c69f45ef459e086"),
ObjectId("5d81171b2c69f45ef459e087")
],
"sku" : "5d81171c2c69f45ef459e0af"
},
And my Category Document is:
{
"_id" : ObjectId("5d81171b2c69f45ef459e087"),
"name" : "Art",
"description" : "These items are our artsy options.",
"created_at" : ISODate("2019-09-17T18:25:47.196+01:00")
},
My aim is to perform aggregation on the Product Document in order to count the number of items within each Category. So I have the Category "Art", I need to count the products are in the "Art" Category:
My current aggregate:
db.product.aggregate(
{ $unwind : "$categories" },
{
$group : {
"_id" : { "name" : "$name" },
"doc" : { $push : { "category" : "$categories" } },
}
},
{ $unwind : "$doc" },
{
$project : {
"_id" : 0,
"name" : "$name",
"category" : "$doc.category"
}
},
{
$group : {
"_id" : "$category",
"name": { "$first": "$name" },
"items_in_cat" : { $sum : 1 }
}
},
{ "$sort" : { "items_in_cat" : -1 } },
)
Which does actually work but not as I need:
{
"_id" : ObjectId("5d81171b2c69f45ef459e082"),
"name" : null, // Why is the name of the category no here?
"items_in_cat" : 4
},
As we can see the name is null. How can I aggregate the output to be:
{
"_id" : ObjectId("5d81171b2c69f45ef459e082"),
"name" : "Art",
"items_in_cat" : 4
},

Answer 1

We need to use $lookup to fetch the name from Category collection.
The following query can get us the expected output:
db.product.aggregate([
{
$unwind:"$categories"
},
{
$group:{
"_id":"$categories",
"items_in_cat":{
$sum:1
}
}
},
{
$lookup:{
"from":"category",
"let":{
"id":"$_id"
},
"pipeline":[
{
$match:{
$expr:{
$eq:["$_id","$$id"]
}
}
},
{
$project:{
"_id":0,
"name":1
}
}
],
"as":"categoryLookup"
}
},
{
$unwind:{
"path":"$categoryLookup",
"preserveNullAndEmptyArrays":true
}
},
{
$project:{
"_id":1,
"name":{
$ifNull:["$categoryLookup.name","NA"]
},
"items_in_cat":1
}
}
]).pretty()
Data set:
Collection: product
{
"_id" : ObjectId("5d81171c2c69f45ef459e0af"),
"type" : "T-Shirt",
"name" : "Panda",
"description" : "Panda's are cool.",
"image" : ObjectId("5d81171c2c69f45ef459e0ad"),
"created_at" : ISODate("2019-09-17T17:25:48.026Z"),
"is_featured" : false,
"sizes" : [
"XS",
"S",
"M",
"L",
"XL"
],
"tags" : [ ],
"pricing" : {
"price" : 26,
"sale_price" : 8
},
"categories" : [
ObjectId("5d81171b2c69f45ef459e086"),
ObjectId("5d81171b2c69f45ef459e087")
],
"sku" : "5d81171c2c69f45ef459e0af"
}
Collection: category
{
"_id" : ObjectId("5d81171b2c69f45ef459e086"),
"name" : "Art",
"description" : "These items are our artsy options.",
"created_at" : ISODate("2019-09-17T17:25:47.196Z")
}
{
"_id" : ObjectId("5d81171b2c69f45ef459e087"),
"name" : "Craft",
"description" : "These items are our artsy options.",
"created_at" : ISODate("2019-09-17T17:25:47.196Z")
}
Output:
{
"_id" : ObjectId("5d81171b2c69f45ef459e087"),
"items_in_cat" : 1,
"name" : "Craft"
}
{
"_id" : ObjectId("5d81171b2c69f45ef459e086"),
"items_in_cat" : 1,
"name" : "Art"
}