Mongoose: how to use aggregate and find together - node.js

Question

How can I use aggregate and find together in Mongoose?
i.e I have the following schema:
const schema = new Mongoose.Schema({
created: { type: Date, default: Date.now() },
name: { type: String, default: 'development' }
followers: [{ type: Mongoose.Schema.ObjectId, ref: 'Users'}]
...
})
export default Mongoose.model('Locations', schema)
How can I query the users with only the fields name and followers_count.
followers_count: the length of followers .
There, I know we can use select to get only the field name.
How can we get the count of followers?

Answer 1

For MongoDB 3.6 and greater, use the $expr operator which allows the use of aggregation expressions within the query language:
var followers_count = 30;
db.locations.find({
"$expr": {
"$and": [
{ "$eq": ["$name", "development"] },
{ "$gte": [{ "$size": "$followers" }, followers_count ]}
]
}
});
For non-compatible versions, you can use both the $match and $redact pipelines to query your collection. For example, if you want to query the locations collection where the name is 'development' and followers_count is greater than 30, run the following aggregate operation:
const followers_count = 30;
Locations.aggregate([
{ "$match": { "name": "development" } },
{
"$redact": {
"$cond": [
{ "$gte": [ { "$size": "$followers" }, followers_count ] },
"$$KEEP",
"$$PRUNE"
]
}
}
]).exec((err, locations) => {
if (err) throw err;
console.log(locations);
})
or within a single pipeline as
Locations.aggregate([
{
"$redact": {
"$cond": [
{
"$and": [
{ "$eq": ["$name", "development"] },
{ "$gte": [ { "$size": "$followers" }, followers_count ] }
]
},
"$$KEEP",
"$$PRUNE"
]
}
}
]).exec((err, locations) => {
if (err) throw err;
console.log(locations);
})
The above will return the locations with just the _id references from the users. To return the users documents as means to "populate" the followers array, you can then append the $lookup pipeline.
If the underlying Mongo server version is 3.4 and newer, you can run the pipeline as
let followers_count = 30;
Locations.aggregate([
{ "$match": { "name": "development" } },
{
"$redact": {
"$cond": [
{ "$gte": [ { "$size": "$followers" }, followers_count ] },
"$$KEEP",
"$$PRUNE"
]
}
},
{
"$lookup": {
"from": "users",
"localField": "followers",
"foreignField": "_id",
"as": "followers"
}
}
]).exec((err, locations) => {
if (err) throw err;
console.log(locations);
})
else you would need to $unwind the followers array before applying $lookup and then regroup with $group pipeline after that:
let followers_count = 30;
Locations.aggregate([
{ "$match": { "name": "development" } },
{
"$redact": {
"$cond": [
{ "$gte": [ { "$size": "$followers" }, followers_count ] },
"$$KEEP",
"$$PRUNE"
]
}
},
{ "$unwind": "$followers" },
{
"$lookup": {
"from": "users",
"localField": "followers",
"foreignField": "_id",
"as": "follower"
}
},
{ "$unwind": "$follower" },
{
"$group": {
"_id": "$_id",
"created": { "$first": "$created" },
"name": { "$first": "$name" },
"followers": { "$push": "$follower" }
}
}
]).exec((err, locations) => {
if (err) throw err;
console.log(locations);
})

Answer 2

You can use as the following:
db.locations.aggregate([
{$match:{"your find query"}},
{$project:{"your desired fields"}}
])
In the match you can do stuff like:
{{$match:{name:"whatever"}}
In the project, you can select the fields you want using numbers 0 or 1 like:
{$project:{_id:1,created:0,name:1}}
Which 0 means, do not put and 1 means put.