How to lookup a field in an array of subdocuments in mongoose?

Question

I have an array of review objects like this :

   "reviews": {
        "author": "5e9167c5303a530023bcae42",
        "rate": 5,
        "spoiler": false,
        "content": "This is a comment This is a comment This is a comment.",
        "createdAt": "2020-04-12T16:08:34.966Z",
        "updatedAt": "2020-04-12T16:08:34.966Z"
    },

What I want to achieve is to lookup the author field and get the user data, but the problem is that the lookup I am trying to use only returns this to me:

Code :

 .lookup({
    from: 'users',
    localField: 'reviews.author',
    foreignField: '_id',
    as: 'reviews.author',
  })

Response :

Any way to get the author's data in that field? That's where the author's Id is.

Answer 1

Try to execute below query on your database :

db.reviews.aggregate([
  /** unwind in general is not needed for `$lookup` for if you wanted to match lookup result with specific elem in array is needed */
  {
    $unwind: { path: "$reviews", preserveNullAndEmptyArrays: true },
  },
  {
    $lookup: {
      from: "users",
      localField: "reviews.author",
      foreignField: "_id",
      as: "author", // Pull lookup result into 'author' field
    },
  },
  /** Update 'reviews.author' field in 'reviews' object by checking if   'author' field got a match from 'users' collection.
   * If Yes - As lookup returns an array get first elem & assign(As there will be only one element returned -uniques),
   * If No - keep 'reviews.author' as is */
  {
    $addFields: {
      "reviews.author": {
        $cond: [
          { $ne: ["$author", []] },
          { $arrayElemAt: ["$author", 0] },
          "$reviews.author",
        ],
      },
    },
  },
  /** Group back the documents based on '_id' field & push back all individual 'reviews' objects to 'reviews' array */
  {
    $group: {
      _id: "$_id",
      reviews: { $push: "$reviews" },
    },
  },
]);

Test : MongoDB-Playground

Note : Just in case if you've other fields in document along with reviews that needs to be preserved in output then starting at $group use these stages :

  {
    $group: {
      _id: "$_id",
      data: {
        $first: "$$ROOT"
      },
      reviews: {
        $push: "$reviews"
      }
    }
  },
  {
    $addFields: {
      "data.reviews": "$reviews"
    }
  },
  {
    $project: {
      "data.author": 0
    }
  },
  {
    $replaceRoot: {
      newRoot: "$data"
    }
  }

Test : MongoDB-Playground

Note : Try to keep queries to run on lesser datasets maybe by adding $match as first stage to filter documents & also have proper indexes.