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 expressioninin`.

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 }