MongoDB聚合运算符$toBool详解

{
   $toBool: <expression>
}

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

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" },  //注意：所有的字符串都转换为true
   { _id: 6, item: "nougat", shipped: ""  }       //注意：所有的字符串都转换为true
] )

//定义shippedConversionStage阶段，添加转换后的发货标志字段`convertedShippedFlag`
//因为所有的字符串都会被转换为true，所以要对字符串"false"做个特殊处理
shippedConversionStage = {
   $addFields: {
      convertedShippedFlag: {
         $switch: {
            branches: [
              { case: { $eq: [ "$shipped", "false" ] }, then: false } ,
              { case: { $eq: [ "$shipped", "" ] }, then: false }
            ],
            default: { $toBool: "$shipped" }
        }
      }
   }
};
// 定义文档过滤阶段，过滤出没有发货的订单
unshippedMatchStage = {
   $match: { "convertedShippedFlag": false }
};
db.orders.aggregate( [
  shippedConversionStage,
  unshippedMatchStage
] )