db.command.aggregate.let

1. Operator Description

Feature: Define custom variables and use them in specified expressions, returning the result of the expression.

Syntax: db.command.aggregate.let({ vars: { <variable1>: <variable_expression>, <variable2>: <variable_expression>, ... }, in: <result_expression> })

2. Operator Parameters

Field	Type	Required	Description
-	Object	Yes	in the form of { vars: {xxx:xxx, ...}, in: xxx}, parameters are detailed below

Parameter description:

Multiple variables can be defined in vars, whose values are computed by variable expressions. These defined variables are only accessible within the result expression in in.

When accessing custom variables in the in result expression, prefix the variable name with a double dollar sign ($$) and enclose it in quotes.

3. Sample Code

Suppose the records in the collection goods representing products are as follows: price represents the product price, discount represents the product discount rate, and cost represents the product cost.

{ "cost": -10, "discount": 0.95, "price": 100 }
{ "cost": -15, "discount": 0.98, "price": 1 }
{ "cost": -10, "discount": 1, "price": 10 }

Using let, you can define and compute the actual sales price for each product and assign it to a custom variable priceTotal. Finally, perform a sum operation on priceTotal and cost to obtain the profit for each product.

The code is as follows:

// Sample code in the Cloud Function environment
const tcb = require('@cloudbase/node-sdk')
const app = tcb.init({
  env: 'xxx'
})

const db = app.database()
const $ = db.command.aggregate
const _ = db.command

exports.main = async (event, context) => {
  const res = await db
    .collection('goods')
    .aggregate()
    .project({
      profit: $.let({
        vars: {
          priceTotal: $.multiply(['$price', '$discount'])
        },
        in: $.sum(['$$priceTotal', '$cost'])
      })
    })
    .end()
  console.log(res.data)
}

The returned data result is as follows:

{ "profit": 85 }
{ "profit": -14.02 }
{ "profit": 0 }