$last (array operator)

On this page

Definition

Syntax
Behavior
Example
Learn More

This version of the documentation is archived and no longer supported. View the current documentation to learn how to upgrade your version of MongoDB.

Definition

$last: New in version 4.4.
Returns the last element in an array.
Note
Disambiguation
This page describes the $last array operator. For the $last aggregation accumulator, see $last (aggregation accumulator).

Syntax

The $last operator has the following syntax:

{ $last: <expression> }

The <expression> can be any valid expression as long as it resolves to an array, null, or missing. For more information on expressions, see Expressions.

The $last operator is an alias for the following $arrayElemAt expression:

{ $arrayElemAt: [ <array expression>, -1 ] }

Behavior

Valid Operands

Valid operand for $last must resolve to an array, null, or missing.

If the operand resolves to a non-empty array, $last returns the last element in the array:
If the operand resolves to an empty array [], $last does not return a value.
If the operand is null or missing, $last returns null.

For example, create a test collection example1 with the following documents:

db.example1.insertMany([
     { "_id" : 1, "x" : [ 1, 2, 3 ] },      // Non-empty array
     { "_id" : 2, "x" : [ [ ] ] },          // Non-empty array
     { "_id" : 3, "x" : [ null ] },         // Non-empty array
     { "_id" : 4, "x" : [ ] },              // Empty array
     { "_id" : 5, "x" : null },             // Is null
     { "_id" : 6 }                          // Is Missing
])

Then, the following adds a new field lastElem whose value is derived from applying the $last operator to the x field:

db.example1.aggregate([
   { $addFields: { lastElem: { $last: "$x" } } }
])

The operator returns the following documents:

{ "_id" : 1, "x" : [ 1, 2, 3 ], "lastElem" : 3 }
{ "_id" : 2, "x" : [ [ ] ], "lastElem" : [ ] }
{ "_id" : 3, "x" : [ null ], "lastElem" : null }
{ "_id" : 4, "x" : [ ] }                         // No output
{ "_id" : 5, "x" : null, "lastElem" : null }
{ "_id" : 6, "lastElem" : null }

Invalid Operands

If the operand does not resolve to an array, null, or missing, the aggregation operation as a whole errors.

For example, create a test collection example2 with the following documents:

db.example2.insertMany([
   { "_id" : 1, "x" : [ 1, 2, 3 ] },
   { "_id" : 2, "x" : 2 },             // x is not an array/null or missing
])

Then, the following aggregation operation returns an error because of the { "_id" : 2, "x" : 2 } document:

db.example2.aggregate( { $addFields: { lastElem: { $last: "$x" } } } )

That is, the operation returns the following:

2020-01-20T21:12:26.713-05:00 E  QUERY    [js] uncaught exception: Error: command failed: {
   "ok" : 0,
   "errmsg" : "$last's argument must be an array, but is double",
   "code" : 28689,
   "codeName" : "Location28689"
} : aggregate failed :

Example

Create a sample collection runninglog with the following documents:

db.runninglog.insertMany([
   { "_id" : 1, "team" : "Anteater", log: [ { run: 1, distance: 8 }, { run2: 2, distance: 7.5 }, { run: 3, distance: 9.2 } ] },
   { "_id" : 2, "team" : "Bears", log: [ { run: 1, distance: 18 }, { run2: 2, distance: 17 }, { run: 3, distance: 16 } ] },
   { "_id" : 3, "team" : "Cobras", log: [ { run: 1, distance: 2 } ] }
])

The following aggregation uses the $first and $last operator on the log array to retrieve the information for the first run and the last run:

db.runninglog.aggregate([
   { $addFields: { firstrun: { $first: "$log" }, lastrun: { $last: "$log" } } }
])

The operation returns the following results:

{ "_id" : 1, "team" : "Anteater", "log" : [ { "run" : 1, "distance" : 8 }, { "run2" : 2, "distance" : 7.5 }, { "run" : 3, "distance" : 9.2 } ],
      "firstrun" : { "run" : 1, "distance" : 8 }, "lastrun" : { "run" : 3, "distance" : 9.2 } }
{ "_id" : 2, "team" : "Bears", "log" : [ { "run" : 1, "distance" : 18 }, { "run2" : 2, "distance" : 17 }, { "run" : 3, "distance" : 16 } ],
      "firstrun" : { "run" : 1, "distance" : 18 }, "lastrun" : { "run" : 3, "distance" : 16 } }
{ "_id" : 3, "team" : "Cobras", "log" : [ { "run" : 1, "distance" : 2 } ],
      "firstrun" : { "run" : 1, "distance" : 2 }, "lastrun" : { "run" : 1, "distance" : 2 } }

To calculate the change between the first and the last distances, the following operation uses $cond and $size operators to calculate the difference (i.e. $subtract) the two distances if there are two or more elements in the log array:

db.runninglog.aggregate([
  { $addFields: { firstrun: { $first: "$log" }, lastrun: { $last: "$log" } } },
  { $project: { team: 1, progress:
      {
        $cond: {
           if: { $gt: [ { $size:"$log" }, 1 ] } ,
           then: { $subtract: [ "$lastrun.distance", "$firstrun.distance"] },
           else: "Not enough data." }
      }
  }}
])

The operation returns the following documents:

{ "_id" : 1, "team" : "Anteater", "progress" : 1.1999999999999993 }
{ "_id" : 2, "team" : "Bears", "progress" : -2 }
{ "_id" : 3, "team" : "Cobras", "progress" : "Not enough data." }

Learn More

Back

$last (aggregation)

$let (aggregation)