$toBool (expression operator)（表达式运算符）

Definition定义

$toBool

Converts a value to a boolean.将值转换为布尔值。

$toBool has the following syntax:具有以下语法：

{
   $toBool: <expression>
}

The $toBool takes any valid expression.$toBool接受任何有效表达式。

The $toBool is a shorthand for the following $convert expression:$toBool是以下$convert表达式的简写：

{ $convert: { input: <expression>, to: "bool" } }

Tip

$convert

Behavior行为

The following table lists the input types that can be converted to a boolean:下表列出了可以转换为布尔值的输入类型：

To learn more about data types in MongoDB, see BSON Types.要了解有关MongoDB中数据类型的更多信息，请参阅BSON类型。

The following table lists some conversion to boolean examples:下表列出了一些转换为布尔值的示例：

Example示例

Create a collection orders with the following documents:使用以下文档创建集合orders：

db.orders.insertMany( [
   { _id: 1, item: "apple",  qty: 5, shipped: true },
   { _id: 2, item: "pie",  qty: 10, shipped: 0  },
   { _id: 3, item: "ice cream", shipped: 1 },
   { _id: 4, item: "almonds", qty: 2, shipped: "true" },
   { _id: 5, item: "pecans", shipped: "false" },  // Note: All strings convert to true
   { _id: 6, item: "nougat", shipped: ""  }       // Note: All strings convert to true
] )

The following aggregation operation on the orders collection converts the shipped to a boolean value before finding the unshipped orders:在查找未发货的订单之前，orders集合上的以下聚合操作会将shipped转换为布尔值：

// Define stage to add convertedShippedFlag field with the converted shipped value定义阶段以添加已转换发货标志字段和已转换发货值
// Because all strings convert to true, include specific handling for "false" and ""因为所有字符串都转换为true，所以包括对"false"和""的特定处理

shippedConversionStage = {
   $addFields: {
      convertedShippedFlag: {
         $switch: {
            branches: [
              { case: { $eq: [ "$shipped", "false" ] }, then: false } ,
              { case: { $eq: [ "$shipped", "" ] }, then: false }
            ],
            default: { $toBool: "$shipped" }
        }
      }
   }
};

// Define stage to filter documents and pass only the unshipped orders定义阶段以筛选文档并仅传递未发货的订单

unshippedMatchStage = {
   $match: { "convertedShippedFlag": false }
};

db.orders.aggregate( [
  shippedConversionStage,
  unshippedMatchStage
] )

The operation returns the following document:该操作返回以下文档：

{ "_id" : 2, "item" : "pie", "qty" : 10, "shipped" : 0, "convertedShippedFlag" : false }
{ "_id" : 5, "item" : "pecans", "shipped" : "false", "convertedShippedFlag" : false }
{ "_id" : 6, "item" : "nougat", "shipped" : "", "convertedShippedFlag" : false }