适用对象：
MongoDB vCore
$convert运算符将表达式转换为指定类型的值。 如果输入表达式转换为指定类型失败，$convert运算符也会执行指定的作。
语法
运算符的 $convert 语法为：
{ "$convert": {"input": <expression>, "to": <type>, "format": <binData format>, "onError": <value to return on error>, "onNull": <value to return on null> }
参数
| 参数 | DESCRIPTION |
|---|---|
input |
要转换为指定类型的输入值 |
to |
要将输入值转换为的类型 |
format |
（可选）在将值转换为 binData 或从 binData 转换时，输入或输出的 binData 格式 |
onError |
（可选）将输入转换为指定类型失败时要返回的值 |
onNull |
（可选）当要转换为指定类型的输入值为 null 或缺失时要返回的值 |
例子
请查看 StoreData 数据库中 stores 集合的这个示例文档。
{
"_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
"name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
"location": {
"lat": -89.2384,
"lon": -46.4012
},
"staff": {
"totalStaff": {
"fullTime": 8,
"partTime": 20
}
},
"sales": {
"totalSales": 75670,
"salesByCategory": [
{
"categoryName": "Wine Accessories",
"totalSales": 34440
},
{
"categoryName": "Bitters",
"totalSales": 39496
},
{
"categoryName": "Rum",
"totalSales": 1734
}
]
},
"promotionEvents": [
{
"eventName": "Unbeatable Bargain Bash",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 6,
"Day": 23
},
"endDate": {
"Year": 2024,
"Month": 7,
"Day": 2
}
},
"discounts": [
{
"categoryName": "Whiskey",
"discountPercentage": 7
},
{
"categoryName": "Bitters",
"discountPercentage": 15
},
{
"categoryName": "Brandy",
"discountPercentage": 8
},
{
"categoryName": "Sports Drinks",
"discountPercentage": 22
},
{
"categoryName": "Vodka",
"discountPercentage": 19
}
]
},
{
"eventName": "Steal of a Deal Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 9,
"Day": 21
},
"endDate": {
"Year": 2024,
"Month": 9,
"Day": 29
}
},
"discounts": [
{
"categoryName": "Organic Wine",
"discountPercentage": 19
},
{
"categoryName": "White Wine",
"discountPercentage": 20
},
{
"categoryName": "Sparkling Wine",
"discountPercentage": 19
},
{
"categoryName": "Whiskey",
"discountPercentage": 17
},
{
"categoryName": "Vodka",
"discountPercentage": 23
}
]
}
]
}
示例 1：将 Int 值转换为字符串
db.stores.aggregate([
{
"$match": {
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d"
}
},
{
"$project": {
"fulltimeStaff": "$staff.totalStaff.fullTime",
"fulltimeStaffAsString": {
"$convert": {
"input": "$staff.totalStaff.fullTime",
"to": "string"
}
}
}
}])
此查询返回以下结果：
{
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d",
"fulltimeStaff": 3,
"fulltimeStaffAsString": "3"
}
示例 2：将 Int 值转换为布尔值
db.stores.aggregate([
{
"$match": {
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d"
}
},
{
"$project": {
"fulltimeStaff": "$staff.totalStaff.fullTime",
"fulltimeStaffAsBool": {
"$convert": {
"input": "$staff.totalStaff.fullTime",
"to": "bool"
}
}
}
}])
此查询返回以下结果：
{
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d",
"fulltimeStaff": 3,
"fulltimeStaffAsBool": true
}
示例 3：将 Int 值转换为十进制值
db.stores.aggregate([
{
"$match": {
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d"
}
},
{
"$project": {
"fulltimeStaff": "$staff.totalStaff.fullTime",
"fulltimeStaffAsDecimal": {
"$convert": {
"input": "$staff.totalStaff.fullTime",
"to": "decimal"
}
}
}
}])
此查询返回以下结果：
{
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d",
"fulltimeStaff": 3,
"fulltimeStaffAsDecimal": "Decimal128('3')"
}
示例 4：将 Int 值转换为 Long 值
db.stores.aggregate([
{
"$match": {
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d"
}
},
{
"$project": {
"fulltimeStaff": "$staff.totalStaff.fullTime",
"fulltimeStaffAsLong": {
"$convert": {
"input": "$staff.totalStaff.fullTime",
"to": "long"
}
}
}
}])
此查询返回以下结果：
{
"_id": "b0107631-9370-4acd-aafa-8ac3511e623d",
"fulltimeStaff": 3,
"fulltimeStaffAsLong": "Long('3')"
}