MongoDB Aggregation, Mongodb Query

Question

I am using Nodejs and MongoDB with expressjs and mongoose library, creating a Blog API having users, articles & comments schema. Below are schemas that I use.

const UsersSchema = new mongoose.Schema({
    username:        { type: String },
    email:           { type: String },
    date_created:    { type: Date },
    last_modified:   { type: Date }
});    




const ArticleSchema = new mongoose.Schema({
    id:              { type: String, required: true },
    text:            { type: String, required: true }, 
    posted_by:       { type: Schema.Types.ObjectId, ref: 'User', required: true },
    images:          [{ type: String }],
    date_created:    { type: Date },
    last_modified:   { type: Date }
});




const CommentSchema = new mongoose.Schema({
    id:             { type: String, required: true },
    commented_by:   { type: Schema.Types.ObjectId, ref: 'User', required: true },
    article:        { type: Schema.Types.ObjectId, ref: 'Article' },
    text:           { type: String, required: true },
    date_created:   { type: Date },
    last_modified:  { type: Date } 
});

Ashh · Accepted Answer · 2019-01-24 18:28:43Z

2

You can use below $lookup aggregation from mongodb 3.6 and above

Article.aggregate([
  { "$match": { "posted_by": mongoose.Types.ObjectId(id) } },
  { "$lookup": {
    "from": Comment.collection.name,
    "let": { "id": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$article", "$$id" ] } } }
    ],
    "as": "comments"
  }},
  { "$addFields": {
    "comments_no": { "$size": "$comments" },
    "hasCommented": { "$in": [mongoose.Types.ObjectId(id), "$comments.commented_by"] }
  }}
])

answered Jan 24, 2019 at 18:28

Ashh

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4 Comments

Ashh Over a year ago

Ok then remove the $match stage if you want to get the data for all the users. it is not returning the exact number of comment It should not happen

Gor Kotikyan Over a year ago

Yes, it is working but when MongoDB Replicaset is enabled. if it is enabled, you will not be able to use some aggregation functionality including $lookup so be careful.

Stennie Over a year ago

@GorKotikyan The restriction on $lookup (as at MongoDB 4.0) is that the from collection must be unsharded and in the same database -- you can still use the $lookup command in a replica set or sharded environment. For expanded support there is a relevant feature request you can watch/upvote in the MongoDB issue: SERVER-29159: Add $lookup support for sharded collections.

byal Over a year ago

Hello, @AnthonyWinzlet can you help me on this question? stackoverflow.com/q/54376551/9159002

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