Aggregation filter and lookup on Mongodb - mongodb

Question

My first collection employeecategory is like below;
[{
name: "GARDENING"
},
{
name: "SECURITY"
},
{
name: "PLUMBER"
}
]
My second collection complaints is like below;
[{
communityId: 1001,
category: "SECURITY",
//other fields
}, {
communityId: 1001,
category: "GARDENING",
//other fields
}]
I am trying to join above tables and get the below result;
[{
"count": 1,
"name": "GARDENING"
}, {
"count": 1,
"name": "SECURITY"
}, {
"count": 0,
"name": "PLUMBER"
}]
Even if there are no records in collection 2 I need count. I tried below aggregation but didn't worked. If I removed match condition it is working but I need to filter on community id. Could some please suggest best way to do achieve this. Mongo DB version is 3.4.0
db.employeecategory.aggregate(
[{
$match: {
"complaints.communityId": 1001
}
}, {
"$lookup": {
from: "complaints",
localField: "name",
foreignField: "category",
as: "embeddedData"
}
}]
)

Answer 1

It was not possible to achieve both filtering for communityId = 1001 and grouping without losing count = 0 category in a single aggregation. The way to do it is first start from complaints collection, and filter the communityId = 1001 objects, and create a temp collection with it. Then from employeecategory collection, $lookup to join with that temp collection, and $group with name, you will have your result at this point, then drop the temp table.
// will not modify complaints document, will create a filtered temp document
db.complaints.aggregate(
[{
$match: {
communityId: 1001
}
},
{
$out: "temp"
}
]
);
// will return the answer that is requested by OP
db.employeecategory.aggregate(
[{
$lookup: {
from: "temp",
localField: "name",
foreignField: "category",
as: "array"
}
}, {
$group: {
_id: "$name",
count: {
$sum: {
$size: "$array"
}
}
}
}]
).pretty();
db.temp.drop(); // to get rid of this temporary collection
will result;
{ _id: "PLUMBER", count: 0},
{ _id: "SECURITY", count: 2},
{ _id: "GARDENING", count: 1}
for the test data I've had;
db.employeecategory.insertMany([
{ name: "GARDENING" },
{ name: "SECURITY" },
{ name: "PLUMBER" }
]);
db.complaints.insertMany([
{ category: "GARDENING", communityId: 1001 },
{ category: "SECURITY", communityId: 1001 },
{ category: "SECURITY", communityId: 1001 },
{ category: "SECURITY", communityId: 1002 }
]);