Querying a referenced document in MongoDB using Mongoose - mongodb

Question

This is a document in the collection BlogPosts:
{
_id: ObjectId("..."),
post_title: "Hello World!",
post_body: "",
comments: [
{ user_id: ObjectId("123"), body: "nice post!" },
{ user_id: ObjectId("456"), body: "awesome!" },
]
}
I would like to display comments with the user's first name, which is found in the referenced document in the Users collection:
{
_id: ObjectId("123"),
first_name: "Marion",
last_name: "Smith",
email_address: "marion#example.com",
password: "..."
}
Is there a way to retrieve the BlogPosts document while including first_name from this referenced data?
For example, I'm looking for an output like this (each comment has a first name):
{
_id: ObjectId("..."),
post_title: "Hello World!",
post_body: "",
comments: [
{ user_id: ObjectId("..."), first_name: "Marion", body: "nice post!" },
{ user_id: ObjectId("..."), first_name: "Margaret", body: "awesome!" },
]
}
I'm using Mongoose.

Answer 1

You can use below aggregation
db.collection.aggregate([
{ "$unwind": "$comments" },
{ "$lookup": {
"from": "users",
"let": { "userId": "$comments.user_id" },
"pipeline": [{ "$match": { "$expr": { "$eq": ["$$userId", "$_id"] } } }],
"as": "user"
}},
{ "$addFields": {
"comments.first_name": { "$arrayElemAt": ["$user.first_name", 0] }
}},
{ "$group": {
"_id": "$_id",
"comments": { "$push": "$comments" },
"post_title": { "$first": "$post_title" },
"post_body": { "$first": "$post_body" }
}}
])

Answer 2

I've since found a more straightforward approach, using just Populate.
BlogPosts
.findOne({_id: req.params.id})
.populate('comments.user_id', ['first_name', 'last_name'])
.then(post => console.log(post))
In the schema for BlogPosts, a ref should be defined for the comments.user_id field:
const User = require('./User.model.js');
const blogPostSchema = new Schema({
post_title: { type: String },
post_body: { type: String },
comments: [{
user_id: {
type: Schema.ObjectId,
ref: 'User' <-------- here
}
}]
});
const BlogPost = mongoose.model('BlogPost', blogPostSchema);
module.exports = BlogPost;