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Obvious_Grapefruit
Obvious_Grapefruit

Reputation: 890

Querying a referenced document in MongoDB using Mongoose

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: "[email protected]",
    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.

Upvotes: 1

Views: 603

Answers (1)

Ashh
Ashh

Reputation: 46461

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" }
  }}
])

Upvotes: 2

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