mongoose add a field to a document from another document

Question

I want to get categories by limit, offset, orderBy viewers from VideoSchema

const CategoriesSchema = new Schema<ICategories>({
  name: String,
  image: String,
});

const VideoSchema = new Schema<IVideo>({
  videoSource: String,
  // from CategoriesSchema
  category: {
    name: String,
    image: String,
  },
  viewers: Number,
});

What i do to get what i want:

const resolver = async ({ args: { limit, offset } }) => {
      // there are 60k+ categories so request will be long
      const categories = await CategoriesModel.find();
      for (let i = 0; i < categories.length; i++) {
        const streams = await VideosModel.find({ 'category.name': categories[i].name });
        categories[i] = {
          ...categories[i],
          // count viewers of current video
          viewers: streams.reduce((acc, curr) => acc + curr.viewers, 0),
        };
      }
      // sort by viewers, slice array by limit and offset
      return categories.sort(({ viewers: viewersA = 0 }, { viewers: viewersB = 0 }) => viewersB - viewersA).slice(offset, limit);
  },

I understand that request is not optimized, but I can't figure out how to do request by limit and offset and sort it by current viewers count. I know that in relational database like mysql i can use left join for that case, but don't understand how to do something like that with mongoose

Clean up:

There is Categories list, each category have number of viewers from VideoSchema (viewers field). If there is no video for that category then should have 0 viewers or if we have video for that category we print number of viewers from 0 to 2^32

Also I have changes in VideoSchema, for now its with ref to Categories

const VideoSchema = new Schema<IVideo>({
  videoSource: String,
  // from CategoriesSchema
  category: {
    ref: 'Categories'
  },
  viewers: Number,
});

Answer 1

If I understand correctly, you want for each category , to add the sum of viewers from the relevant videos , sort and slice.

On mongoDB you can easily do it all in one query. Since you want to get categories with 0 viewers as well, we will start from the category collection:

1. $lookup to get all videos per category and sum the number of viewers per each category inside the $lookup pipeline, to minimize data transformation between the collections.
2. Format the response and handle cases of no video data
3. $sort by number of viewers , $skip and $limit - for the slicing. Add the _id to the $sort as a second option, in order to keep it sorted even for 0 viewers categories.

db.category.aggregate([
  {
    $lookup: {
      from: "video",
      let: {category_id: "$_id"},
      pipeline: [
        {$match: {$expr: {$eq: ["$category", "$$category_id"]}}},
        {$group: {_id: 0, viewers: {$sum: "$viewers"}}}
      ],
      as: "viewers"
    }
  },
  {$set: {viewers: {$ifNull: [{$first: "$viewers.viewers" }, 0]}}},
  {$sort: {viewers: -1, _id: 1}},
  {$skip: offset},
  {$limit: limit}
])

See how it works on the playground example