How to use $lookup as INNER JOIN in MongoDB Aggregation? - mongodb

Question

I have used $lookup in my aggregate query.
But as I am seeing it works as LEFT OUTER JOIN.
I want to fetch exact matches document(INNER JOIN) with $lookup.
Is there any way to get it done?
This is my inventory collection:
/* 1 */
{
"_id" : 1,
"sku" : "abc",
"description" : "product 1",
"instock" : 120
}
/* 2 */
{
"_id" : 2,
"sku" : "def",
"description" : "product 2",
"instock" : 80
}
/* 3 */
{
"_id" : 3,
"sku" : "ijk",
"description" : "product 3",
"instock" : 60
}
/* 4 */
{
"_id" : 4,
"sku" : "jkl",
"description" : "product 4",
"instock" : 70
}
/* 5 */
{
"_id" : 5,
"sku" : null,
"description" : "Incomplete"
}
This is my orders collection
/* 1 */
{
"_id" : 1,
"item" : "abc",
"price" : 12,
"quantity" : 2
}
/* 2 */
{
"_id" : 2,
"item" : "jkl",
"price" : 20,
"quantity" : 1
}
/* 3 */
{
"_id" : 10,
"item" : "jklw",
"price" : 20,
"quantity" : 1
}
And this is query
db.getCollection('inventory').aggregate([
{
$lookup:
{
from: "orders",
localField: "sku",
foreignField: "item",
as: "inventory_docs"
}
}
])
In this query I am getting all the inventory's document matches with orders documents
Expected Result
/* 1 */
{
"_id" : 1,
"sku" : "abc",
"description" : "product 1",
"instock" : 120,
"inventory_docs" : [
{
"_id" : 1,
"item" : "abc",
"price" : 12,
"quantity" : 2
}
]
}
/* 2 */
{
"_id" : 4,
"sku" : "jkl",
"description" : "product 4",
"instock" : 70,
"inventory_docs" : [
{
"_id" : 2,
"item" : "jkl",
"price" : 20,
"quantity" : 1
}
]
}

Answer 1

Just add the $match pipeline stage which skips documents with empty inventory_docs field. There no other way to achieve that.
Query:
db.getCollection('inventory').aggregate([
{
$lookup: {
from: "orders",
localField: "sku",
foreignField: "item",
as: "inventory_docs"
}
},
{
$match: {
"inventory_docs": {$ne: []}
}
}
])
Result:
{
"_id" : 1.0,
"sku" : "abc",
"description" : "product 1",
"instock" : 120.0,
"inventory_docs" : [
{
"_id" : 1.0,
"item" : "abc",
"price" : 12.0,
"quantity" : 2.0
}
]
}
{
"_id" : 4.0,
"sku" : "jkl",
"description" : "product 4",
"instock" : 70.0,
"inventory_docs" : [
{
"_id" : 2.0,
"item" : "jkl",
"price" : 20.0,
"quantity" : 1.0
}
]
}