Mongodb aggregate pipeline to return multiple fields with $lookup from array

Question

I'm trying to get a list of sorted comments by createdAt from a Post doc where an aggregate pipeline would be used to populate the owner of a comment in comments field with displayName and profilePhoto fields.

Post Schema:

{
  _owner: { type: Schema.Types.ObjectId, ref: 'User', required: true },
  ...
  comments: [
    {
      _owner: { type: Schema.Types.ObjectId, ref: 'User' },
      createdAt: Number,
      body: { type: String, maxlength: 200 }
    }
  ]
}

User schema:

{
  _id: '123abc'
  profilePhoto: String,
  displayName: String,
  ...
 }

What I want to return:

[
  {
    "_id": "5bb5e99e040bf10b884b9653",
    "_owner": {
        "_id": "5bb51a97fb250722d4f5d5e1",
        "profilePhoto": "https://...",
        "displayName": "displayname"
    },
    "createdAt": 1538648478544,
    "body": "Another comment"
  },
  {
    "_id": "5bb5e96686f1973274c03880",
    "_owner": {
        "_id": "5bb51a97fb250722d4f5d5e1",
        "profilePhoto": "https://...",
        "displayName": "displayname"
    },
    "createdAt": 1538648422471,
    "body": "A new comment"
  }
]

I have some working code that goes from aggregate to get sorted comments first, then I populate separately but I want to be able to get this query just by using aggregate pipeline.

Current solution looks like this:

  const postComments = await Post.aggregate([
    { $match: { _id: mongoose.Types.ObjectId(postId) } },
    { $unwind: '$comments' },
    { $limit: 50 },
    { $skip: 50 * page },
    { $sort: { 'comments.createdAt': -1 } },
    {$replaceRoot: {newRoot: '$comments'}},
    {
      $project: {
        _owner: 1,
        createdAt: 1,
        body: 1
      }
    }
  ]);

  await Post.populate(postComments, {path: 'comments._owner', select: 'profilePhoto displayName' } )

Ashh · Accepted Answer · 2018-10-04 16:29:21Z

1

You can try below aggregation

const postComments = await Post.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(postId) } },
  { "$unwind": "$comments" },
  { "$lookup": {
    "from": "users",
    "localField": "comments._owner",
    "foreignField": "_id",
    "as": "comments._owner"
  }},
  { "$unwind": "$comments._owner" },
  { "$replaceRoot": { "newRoot": "$comments" }},
  { "$sort": { "createdAt": -1 } }
  { "$limit": 50 }
])

answered Oct 4, 2018 at 16:29

Ashh

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