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Aggregation Pipeline Stages

$setWindowFields (aggregation)

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  • Definition
  • Syntax
  • Behavior
  • Window Operators
  • Restrictions
  • Examples

Definition

$setWindowFields

New in version 5.0.

Performs operations on a specified span of documents in a collection, known as a window, and returns the results based on the chosen window operator.

For example, you can use the $setWindowFields stage to output the:

  • Difference in sales between two documents in a collection.

  • Sales rankings.

  • Cumulative sales totals.

  • Analysis of complex time series information without exporting the data to an external database.

Syntax

The $setWindowFields stage syntax:

{
   $setWindowFields: {
      partitionBy: <expression>,
      sortBy: {
         <sort field 1>: <sort order>,
         <sort field 2>: <sort order>,
         ...,
         <sort field n>: <sort order>
      },
      output: {
         <output field 1>: {
            <window operator>: <window operator parameters>,
            window: {
               documents: [ <lower boundary>, <upper boundary> ],
               range: [ <lower boundary>, <upper boundary> ],
               unit: <time unit>
            }
         },
         <output field 2>: { ... },
         ...
         <output field n>: { ... }
      }
   }
}

The $setWindowFields stage takes a document with these fields:

Field
Necessity
Description
Optional

Specifies an expression to group the documents. In the $setWindowFields stage, the group of documents is known as a partition. Default is one partition for the entire collection.

Required for some operators (see Restrictions)

Specifies the field(s) to sort the documents by in the partition. Uses the same syntax as the $sort stage. Default is no sorting.

Required

Specifies the field(s) to append to the documents in the output returned by the $setWindowFields stage. Each field is set to the result returned by the window operator.

A field can contain dots to specify embedded document fields and array fields. The semantics for the embedded document dotted notation in the $setWindowFields stage are the same as the $addFields and $set stages. See embedded document $addFields example and embedded document $set example.

Optional

Specifies the window boundaries and parameters. Window boundaries are inclusive. Default is an unbounded window, which includes all documents in the partition.

Specify either a documents or range window.

Optional

A window where the lower and upper boundaries are specified relative to the position of the current document read from the collection.

The window boundaries are specified using a two element array containing a lower and upper limit string or integer. Use:

  • The "current" string for the current document position in the output.

  • The "unbounded" string for the first or last document position in the partition.

  • An integer for a position relative to the current document. Use a negative integer for a position before the current document. Use a positive integer for a position after the current document. 0 is the current document position.

See Documents Window Examples.

Optional

A window where the lower and upper boundaries are defined using a range of values based on the sortBy field in the current document.

The window boundaries are specified using a two element array containing a lower and upper limit string or number. Use:

  • The "current" string for the current document position in the output.

  • The "unbounded" string for the first or last document position in the partition.

  • A number to add to the value of the sortBy field for the current document. A document is in the window if the sortBy field value is inclusively within the lower and upper boundaries.

See Range Window Example.

Optional

Specifies the units for time range window boundaries. Can be set to one of these strings:

  • "year"

  • "quarter"

  • "month"

  • "week"

  • "day"

  • "hour"

  • "minute"

  • "second"

  • "millisecond"

If omitted, default numeric range window boundaries are used.

See Time Range Window Examples.

Tip

See also:

Examples

Behavior

The $setWindowFields stage appends new fields to existing documents. You can include one or more $setWindowFields stages in an aggregation operation.

Starting in MongoDB 5.3, you can use the $setWindowFields stage with transactions and the "snapshot" read concern.

Window Operators

These operators can be used with the $setWindowFields stage:

Restrictions

Restrictions for the $setWindowFields stage:

  • Starting in MongoDB 5.1 (and 5.0.4), the $setWindowFields stage cannot be used:

  • sortBy is required for:

  • Range windows require all sortBy values to be numbers.

  • Time range windows require all sortBy values to be dates.

  • Range and time range windows can only contain one sortBy field and the sort must be ascending.

  • You cannot specify both a documents window and a range window.

  • These operators use an implicit window and return an error if you specify a window option:

  • For range windows, only numbers in the specified range are included in the window. Missing, undefined, and null values are excluded.

  • For time range windows:

    • Only date and time types are included in the window.

    • Numeric boundary values must be integers. For example, you can use 2 hours as a boundary but you cannot use 1.5 hours.

  • For empty windows or windows with incompatible values (for example, using $sum on strings), the returned value depends on the operator:

    • For $count and $sum, the returned value is 0.

    • For $addToSet and $push, the returned value is an empty array.

    • For all other operators, the returned value is null.

Examples

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

The following examples use the cakeSales collection.

Documents Window Examples

Use Documents Window to Obtain Cumulative Quantity for Each State

This example uses a documents window in $setWindowFields to output the cumulative cake sales quantity for each state:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            cumulativeQuantityForState: {
               $sum: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output:

    • Sets the cumulativeQuantityForState field to the cumulative quantity for each state, which increases by successive additions to the previous value in the partition.

    • Calculates the cumulative quantity using the $sum operator run in a documents window.

      The window contains documents between an unbounded lower limit and the current document. This means $sum returns the cumulative quantity for the documents between the beginning of the partition and the current document.

In this example output, the cumulative quantity for CA and WA is shown in the cumulativeQuantityForState field:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "cumulativeQuantityForState" : 162 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "cumulativeQuantityForState" : 282 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "cumulativeQuantityForState" : 427 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "cumulativeQuantityForState" : 134 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "cumulativeQuantityForState" : 238 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "cumulativeQuantityForState" : 378 }

Use Documents Window to Obtain Cumulative Quantity for Each Year

This example uses a documents window in $setWindowFields to output the cumulative cake sales quantity for each $year in orderDate:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: { $year: "$orderDate" },
         sortBy: { orderDate: 1 },
         output: {
            cumulativeQuantityForYear: {
               $sum: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: { $year: "$orderDate" } partitions the documents in the collection by $year in orderDate. There are are partitions for 2019, 2020, and 2021.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output:

    • Sets the cumulativeQuantityForYear field to the cumulative quantity for each year, which increases by successive additions to the previous value in the partition.

    • Calculates the cumulative quantity using the $sum operator run in a documents window.

      The window contains documents between an unbounded lower limit and the current document. This means $sum returns the cumulative quantity for the documents between the beginning of the partition and the current document.

In this example output, the cumulative quantity for each year is shown in the cumulativeQuantityForYear field:

{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "cumulativeQuantityForYear" : 134 }
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "cumulativeQuantityForYear" : 296 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "cumulativeQuantityForYear" : 104 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "cumulativeQuantityForYear" : 224 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "cumulativeQuantityForYear" : 145 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "cumulativeQuantityForYear" : 285 }

Use Documents Window to Obtain Moving Average Quantity for Each Year

This example uses a documents window in $setWindowFields to output the moving average for the cake sales quantity:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: { $year: "$orderDate" },
         sortBy: { orderDate: 1 },
         output: {
            averageQuantity: {
               $avg: "$quantity",
               window: {
                  documents: [ -1, 0 ]
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$orderDate" partitions the documents in the collection by $year in orderDate. There are are partitions for 2019, 2020, and 2021.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output:

    • Sets the averageQuantity field to the moving average quantity for each year.

    • Calculates the moving average quantity using the $avg operator run in a documents window.

      The window contains documents between -1 and 0. This means $avg returns the moving average quantity between the document before the current document (-1) and the current document (0) in the partition.

In this example output, the moving average quantity is shown in the averageQuantity field:

{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "averageQuantity" : 134 }
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "averageQuantity" : 148 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "averageQuantity" : 104 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "averageQuantity" : 112 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "averageQuantity" : 145 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "averageQuantity" : 142.5 }

Use Documents Window to Obtain Cumulative and Maximum Quantity for Each Year

This example uses a documents window in $setWindowFields to output the cumulative and maximum cake sales quantity values for each $year in orderDate:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: { $year: "$orderDate" },
         sortBy: { orderDate: 1 },
         output: {
            cumulativeQuantityForYear: {
               $sum: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            },
            maximumQuantityForYear: {
               $max: "$quantity",
               window: {
                  documents: [ "unbounded", "unbounded" ]
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$orderDate" partitions the documents in the collection by $year in orderDate. There are are partitions for 2019, 2020, and 2021.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output:

    • Sets the cumulativeQuantityForYear field to the cumulative quantity for each year.

    • Calculates the cumulative quantity using the $sum operator run in a documents window.

      The window contains documents between an unbounded lower limit and the current document. This means $sum returns the cumulative quantity for the documents between the beginning of the partition and the current document.

    • Sets the maximumQuantityForYear field to the maximum quantity for each year.

    • Calculates the maximum quantity of all the documents using the $max operator run in a documents window.

      The window contains documents between an unbounded lower and upper limit. This means $max returns the maximum quantity for the documents in the partition.

In this example output, the cumulative quantity is shown in the cumulativeQuantityForYear field and the maximum quantity is shown in the maximumQuantityForYear field:

{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134,
  "cumulativeQuantityForYear" : 134, "maximumQuantityForYear" : 162 }
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162,
  "cumulativeQuantityForYear" : 296, "maximumQuantityForYear" : 162 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104,
  "cumulativeQuantityForYear" : 104, "maximumQuantityForYear" : 120 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120,
   "cumulativeQuantityForYear" : 224, "maximumQuantityForYear" : 120 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145,
  "cumulativeQuantityForYear" : 145, "maximumQuantityForYear" : 145 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140,
  "cumulativeQuantityForYear" : 285, "maximumQuantityForYear" : 145 }

Range Window Example

This example uses a range window in $setWindowFields to return the sum of the quantity values of cakes sold for orders within plus or minus 10 dollars of the current document's price value:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { price: 1 },
         output: {
            quantityFromSimilarOrders: {
               $sum: "$quantity",
               window: {
                  range: [ -10, 10 ]
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.

  • sortBy: { price: 1 } sorts the documents in each partition by price in ascending order (1), so the lowest price is first.

  • output sets the quantityFromSimilarOrders field to the sum of the quantity values from the documents in a range window.

    • The window contains documents between a lower limit of -10 and an upper limit of 10. The range is inclusive.

    • $sum returns the sum of quantity values contained in a range of plus or minus 10 dollars of the current document's price value.

In this example output, the sum of the quantity values for documents in the window is shown in the quantityFromSimilarOrders field:

{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "quantityFromSimilarOrders" : 265 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "quantityFromSimilarOrders" : 265 }
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "quantityFromSimilarOrders" : 162 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "quantityFromSimilarOrders" : 244 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "quantityFromSimilarOrders" : 244 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "quantityFromSimilarOrders" : 134 }

Time Range Window Examples

Use a Time Range Window with a Positive Upper Bound

The following example uses a window with a positive upper bound time range unit in $setWindowFields. The pipeline outputs an array of orderDate values for each state that match the specified time range.

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            recentOrders: {
               $push: "$orderDate",
               window: {
                  range: [ "unbounded", 10 ],
                  unit: "month"
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output:

    • Sets the orderDateArrayForState array field to orderDate values for the documents in each state. The array elements are expanded with additions to the previous elements in the array.

    • Uses $push to return an array of orderDate values from the documents in a range window.

  • The window contains documents between an unbounded lower limit and an upper limit set to 10 (10 months after the current document's orderDate value) using a time range unit.

  • $push returns the array of orderDate values for the documents between the beginning of the partition and the documents with orderDate values inclusively in a range of the current document's orderDate value plus 10 months.

In this example output, the array of orderDate values for CA and WA is shown in the recentOrders field:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162,
  "recentOrders" : [ ISODate("2019-05-18T16:09:01Z") ] }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120,
  "recentOrders" : [ ISODate("2019-05-18T16:09:01Z"), ISODate("2020-05-18T14:10:30Z"), ISODate("2021-01-11T06:31:15Z") ] }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145,
  "recentOrders" : [ ISODate("2019-05-18T16:09:01Z"), ISODate("2020-05-18T14:10:30Z"), ISODate("2021-01-11T06:31:15Z") ] }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134,
  "recentOrders" : [ ISODate("2019-01-08T06:12:03Z") ] }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104,
  "recentOrders" : [ ISODate("2019-01-08T06:12:03Z"), ISODate("2020-02-08T13:13:23Z") ] }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140,
  "recentOrders" : [ ISODate("2019-01-08T06:12:03Z"), ISODate("2020-02-08T13:13:23Z"), ISODate("2021-03-20T11:30:05Z") ] }

Use a Time Range Window with a Negative Upper Bound

The following example uses a window with a negative upper bound time range unit in $setWindowFields. The pipeline outputs an array of orderDate values for each state that match the specified time range.

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            recentOrders: {
               $push: "$orderDate",
               window: {
                  range: [ "unbounded", -10 ],
                  unit: "month"
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output:

    • Sets the orderDateArrayForState array field to orderDate values for the documents in each state. The array elements are expanded with additions to the previous elements in the array.

    • Uses $push to return an array of orderDate values from the documents in a range window.

  • The window contains documents between an unbounded lower limit and an upper limit set to -10 (10 months before the current document's orderDate value) using a time range unit.

  • $push returns the array of orderDate values for the documents between the beginning of the partition and the documents with orderDate values inclusively in a range of the current document's orderDate value minus 10 months.

In this example output, the array of orderDate values for CA and WA is shown in the recentOrders field:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162,
  "recentOrders" : [ ] }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120,
  "recentOrders" : [ ISODate("2019-05-18T16:09:01Z") ] }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145,
  "recentOrders" : [ ISODate("2019-05-18T16:09:01Z") ] }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134,
  "recentOrders" : [ ] }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104,
  "recentOrders" : [ ISODate("2019-01-08T06:12:03Z") ] }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140,
  "recentOrders" : [ ISODate("2019-01-08T06:12:03Z"), ISODate("2020-02-08T13:13:23Z") ] }

Tip

See also:

For an additional example about IOT Power Consumption, see the Practical MongoDB Aggregations e-book.

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