$toBool (aggregation)

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Definition定义

$toBool

New in version 4.0.版本4.0中的新功能。

Converts a value to a boolean.

$toBool has the following syntax:语法如下所示：

{
   $toBool: <expression>
}

The $toBool takes any valid expression.

The $toBool is a shorthand for the following $convert expression:

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

See also参阅

$convert

Behavior行为

The following table lists the input types that can be converted to a boolean:

Input TypeBehavior
Boolean No-op. Returns the boolean value.
Double
Returns true if not zero.
Return false if zero.
Decimal
Returns true if not zero.
Return false if zero.
Integer
Returns true if not zero.
Return false if zero.
Long
Returns true if not zero.
Return false if zero.
ObjectId
Returns true.
String
Returns true.
Date
Returns true.

The following table lists some conversion to boolean examples:

Example示例Results结果
{$toBool: false} false
{$toBool: 1.99999} true
{$toBool: NumberDecimal("5")} true
{$toBool: NumberDecimal("0")} false
{$toBool: 100} true
{$toBool: ISODate("2018-03-26T04:38:28.044Z")} true
{$toBool: "false"} true
{$toBool: ""} true
{$toBool: null} null

Example示例

Create a collection orders with the following documents:

db.orders.insert( [
   { _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:

// Define stage to add convertedShippedFlag field with the converted shipped value
// Because all strings convert to true, include specific handling for "false" and ""

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

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

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

The operation returns the following document:

{ "_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 }

Note

If the conversion operation encounters an error, the aggregation operation stops and throws an error. To override this behavior, use $convert instead.