Aggregate Lookup with pipeline and match not working mongodb - mongodb

Question

I have these 2 simple collections:
items:
{
"id" : "111",
"name" : "apple",
"status" : "active"
}
{
"id" : "222",
"name" : "banana",
"status" : "active"
}
inventory:
{
"item_id" : "111",
"qty" : 3,
"branch" : "main"
}
{
"item_id" : "222",
"qty" : 3
}
Now I want to to only return the items with "status" == "active" and with "branch" that exist and is equal to "main" in the inventory collection. I have this code below but it returns all documents, with the second document having an empty "info" array.
db.getCollection('items')
.aggregate([
{$match:{$and:[
{"status":'active'},
{"name":{$exists:true}}
]
}},
{$lookup:{
as:"info",
from:"inventory",
let:{fruitId:"$id"},
pipeline:[
{$match:{
$and:[
{$expr:{$eq:["$item_id","$$fruitId"]}},
{"branch":{$eq:"main"}},
{"branch":{$exists:true}}
]
}
}
]
}}
])
Can anyone give me an idea on how to fix this?

Answer 1

Your code is doing well. I think you only need a $match stage in the last of your pipeline.
db.items.aggregate([
{
$match: {
$and: [
{ "status": "active" },
{ "name": { $exists: true } }
]
}
},
{
$lookup: {
as: "info",
from: "inventory",
let: { fruitId: "$id" },
pipeline: [
{
$match: {
$and: [
{ $expr: { $eq: [ "$item_id", "$$fruitId" ] } },
{ "branch": { $eq: "main" } },
{ "branch": { $exists: true } }
]
}
}
]
}
},
{
"$match": {
"info": { "$ne": [] }
}
}
])
mongoplayground

Answer 2

Query
match
lookup on id/item_id, and match branch with "main" (if it doesn't exists it will be false anyways)
keep only the not empty
*query is almost the same as #YuTing one,but i had written it anyways, so i send it, for the small difference of alternative lookup syntax
Test code here
items.aggregate(
[{"$match":
{"$expr":
{"$and":
[{"$eq":["$status", "active"]},
{"$ne":[{"$type":"$name"}, "missing"]}]}}},
{"$lookup":
{"from":"inventory",
"localField":"id",
"foreignField":"item_id",
"pipeline":[{"$match":{"$expr":{"$eq":["$branch", "main"]}}}],
"as":"inventory"}},
{"$match":{"$expr":{"$ne":["$inventory", []]}}},
{"$unset":["inventory"]}])