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Aggregation Operators

$toBool (aggregation)

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  • Definition
  • Behavior
  • Example

Definition

$toBool

Converts a value to a boolean.

$toBool has the following syntax:

{
   $toBool: <expression>
}

The $toBool takes any valid expression.

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

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

Tip

See also:

$convert

Behavior

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

Input Type
Behavior
Array
Returns true
Binary data
Returns true
Boolean
No-op. Returns the boolean value.
Code
Returns true
Date
Returns true
Decimal
Returns true if not zero
Return false if zero
Double
Returns true if not zero
Return false if zero
Integer
Returns true if not zero
Return false if zero
JavaScript
Returns true
Long
Returns true if not zero
Return false if zero
MaxKey
Returns true
MinKey
Returns true
Null

Returns null

Object
Returns true
ObjectId
Returns true
Regular expression
Returns true
String
Returns true
Timestamp
Returns true

To learn more about data types in MongoDB, see BSON Types.

The following table lists some conversion to boolean examples:

Example
Results
{$toBool: false}
false
{$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

Create a collection orders with the following documents:

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:

// 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.

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$tanh (aggregation)