I want to find the average rating of a Seller, but Seller is in an array of Sellers, how to find average rating of reviews

I have an array of favoriteSellers that contains multiple sellers. I want to find the user's favorite sellers and then find the averageRating, I did it previously for an array of sellers, but this is giving me an issue since the array is inside the array attribute called favoriteSellers, I want to add also the field averageRating since it is necessary for me.

When I retreive my code, the array looks like:

[
  {
    _id: new ObjectId("62a7ce9550094eafc7a61233"),
    user: new ObjectId("6287e4e61df773752aadc286"),
    favoriteSellers: [ new ObjectId("6293210asdce81d9f2ae1685") ],
  }
]

so each seller should have a field of average rating like:

_id: 'favorite_id.....'
user: 'my id',
favoriteSellers:[
{
 _id: 'kjskjhajkhsjk',
averageRating: 4.6
reviews:[.....],
...
},
{
 _id: 'id______hsjk',
averageRating: 2.6
reviews:[.....],
...
},
{
 _id: 'kjid______khsjk....',
averageRating: 3.6
reviews:[.....],
...
}
]

this is my code:

aggregatePipeline.push({
      $match: { user: req.user._id },
    });

aggregatePipeline.push({
  $addFields: {
    ratingSum: {
      $reduce: {
        initialValue: 0,
        input: "$favoriteStylists.reviews",
        in: { $sum: ["$$value", "$$this.rating"] },
      },
    },
  },
});

    //get average of rating ex. seller1 has a 4.5 averageRating field
    aggregatePipeline.push({
      $addFields: {
        averageRating: {
          $cond: [
            { $eq: [{ $size: "favoriteSellers.reviews" }, 0] }, //if it does not have any reviews, then we will just send 0
            0, //set it to 0
            {
              $divide: ["$ratingSum", { $size: "$reviews" }], //else we can divide to get average Rating
            },
          ],
        },
      },
    });

    let favList = await Favorite.aggregate(aggregatePipeline).exec();

Here is the schema for Favorite:

const favoriteSchema = new mongoose.Schema({
  user: {
    type: mongoose.Schema.Types.ObjectId,
    ref: "User",
    unique: true,
  },
  favoriteSellers: [
    //create array of object id, make sure they are unique for user not to add multiple sellers
    {
      type: mongoose.Schema.Types.ObjectId,
      ref: "Seller",
      unique: true,
    },
  ],
});

Here is my Seller schema:

const sellerSchema = new mongoose.Schema({
  user: {
    type: mongoose.Schema.Types.ObjectId,
    ref: "User",
    unique: true,
  },
  business: businessSchema,
  sellerType: [String],
  reviews: [
    {
      by: {
        type: mongoose.Schema.Types.ObjectId,
        ref: "User",
        unique: true,
      },
      title: {
        type: String,
      },
      message: {
        type: String,
      },
      rating: Number,
      imagesUri: [String],
      timestamp: {
        type: Date,
        default: Date.now,
      },
    },
  ],
...
});

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