Mongoose: how to use aggregate and find together

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 ]}        ]     } }); 

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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.

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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); })