AggregateCommand.cond(value: any): Object

Support: Small program 2.7.4, cloud function 0.8.1, Web

The aggregate operator. Evaluate the Boolean expression and return one of the two specified values.

Parameters

value: any

Returns a value

Object

API description

The use of cond is as follows:

cond({ if: <布尔表达式>, then: <真值>, else: <假值>  })

Or:

cond([ <布尔表达式>, <真值>, <假值> ])

In both forms, three parameters (if, then, else) are required.

If the Boolean expression is true, $cond will return the true value of the value, otherwise it will return the value of

The sample code

Suppose the record of the collection items is as follows:

{ "_id": "0", "name": "item-a", "amount": 100 }
{ "_id": "1", "name": "item-b", "amount": 200 }
{ "_id": "2", "name": "item-c", "amount": 300 }

We can use cond to generate a new field discount based on the amount field:

const $ = db.command.aggregate
db.collection('items').aggregate()
  .project({
    name: 1,
    discount: $.cond({
        if: $.gte(['$amount', 200]),
        then: 0.7,
        else: 0.9
    })
  })
  .end()

The output is as follows:

{ "_id": "0", "name": "item-a", "discount": 0.9 }
{ "_id": "1", "name": "item-b", "discount": 0.7 }
{ "_id": "2", "name": "item-c", "discount": 0.7 }

---

AggregateCommand.ifNull(value: Expression[]): Object

Support: Small program 2.7.4, cloud function 0.8.1, Web

The aggregate operator. Evaluates a given expression and returns an alternative value if the expression results in null, undefined, or does not exist;

Parameters

value: Expression[]

s slt;expressions>, slt;alternative values s/he

Returns a value

Object

API description

IfNull is used in the following form:

ifNull([ <表达式>, <替代值> ])

The sample code

Suppose the record of the collection items is as follows:

{ "_id": "0", "name": "A", "description": "这是商品A" }
{ "_id": "1", "name": "B", "description": null }
{ "_id": "2", "name": "C" }

We can use ifNull to add an alternative value to a document that does not have a desc field, or a document where the desc field is null.

const $ = db.command.aggregate
db.collection('items').aggregate()
  .project({
    _id: 0,
    name: 1,
    description: $.ifNull(['$description', '商品描述空缺'])
  })
  .end()

The output is as follows:

{ "name": "A", "description": "这是商品A" }
{ "name": "B", "description": "商品描述空缺" }
{ "name": "C", "description": "商品描述空缺" }

---

AggregateCommand.switch(value: any): Object

Support: Small program 2.7.4, cloud function 0.8.1, Web

The aggregate operator. The return value is calculated based on the given switch-case-default,

Parameters

value: any

Returns a value

Object

API description

The use of switch is as follows:

switch({
    branches: [
        case: <表达式>, then: <表达式>,
        case: <表达式>, then: <表达式>,
        ...
    ],
    default: <表达式>
})

The sample code

Suppose the record of the collection items is as follows:

{ "_id": "0", "name": "item-a", "amount": 100 }
{ "_id": "1", "name": "item-b", "amount": 200 }
{ "_id": "2", "name": "item-c", "amount": 300 }

We can use switch to generate a new field discount based on the amount field:

const $ = db.command.aggregate
db.collection('items').aggregate()
  .project({
    name: 1,
    discount: $.switch({
        branches: [
            { case: $.gt(['$amount', 250]), then: 0.8 },
            { case: $.gt(['$amount', 150]), then: 0.9 }
        ],
        default: 1
    })
  })
  .end()

The output is as follows:

{ "_id": "0", "name": "item-a", "discount": 1 }
{ "_id": "1", "name": "item-b", "discount": 0.9 }
{ "_id": "2", "name": "item-c", "discount": 0.8 }