How to aggregate a nested array using MongoDB?

Question

I tried to use aggregate to find out each product's monthly sales in my order, but I ran into a problem.

Here's my data structures.

Order.model.ts

const OrderSchema: Schema = new Schema(
  {
    userId: {
      type: String,
      require: true,
    },
    products: [
      {
        product: {
          _id: {
            type: String,
          },
          title: {
            type: String,
          },
          desc: {
            type: String,
          },
          img: {
            type: String,
          },
          categories: {
            type: Array,
          },
          price: {
            type: Number,
          },
          createdAt: {
            type: String,
          },
          updatedAt: {
            type: String,
          },
          size: {
            type: String,
          },
          color: {
            type: String,
          },
        },
        quantity: {
          type: Number,
          default: 1,
        },
      },
    ],
    quantity: {
      type: Number,
    },
    total: {
      type: Number,
    },
    address: {
      type: String,
      require: true,
    },
    status: {
      type: String,
      default: 'pending',
    },
  },
  { timestamps: true },
);

Order-service.ts

public async getIncome(productId?: string) {
    const date = new Date();
    const lastMonth = new Date(date.setMonth(date.getMonth() - 1));
    const previousMonth = new Date(new Date().setMonth(lastMonth.getMonth() - 1));
    //const lastYear = new Date(date.setFullYear(date.getFullYear() - 1));
    const income = await this.order.aggregate([
      {
        $match: {
          createdAt: { $gte: lastMonth },
          ...(productId && {
            products: { $elemMatch: { product: { _id: productId } } },
          }),
        },
      },
      {
        $project: {
          month: { $month: '$createdAt' },
          sales: '$total',
        },
      },
      {
        $group: {
          _id: '$month',
          total: { $sum: '$sales' },
        },
      },
    ]);
    return income;
  }

When I calculate whole sales without productId, it went well, I tried to use elemMatch to find productId, but it won't work, did I miss something?

Answer 1

Try $unwind "products" array first, then apply $match :

const income = await this.order.aggregate([
    {
        $unwind: '$products'
    },
    {
        $match: {
            createdAt: { $gte: lastMonth },
            product: { _id: productId },
        },
    },
    {
        $project: {
            month: { $month: '$createdAt' },
            sales: '$total',
        },
    },
    {
        $group: {
            _id: '$month',
            total: { $sum: '$sales' },
        },
    },
]);