$first (array operator)
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Definition
$first
New in version 4.4.
Returns the first element in an array.
Note
Disambiguation
This page describes the
$first
array operator. For the$first
aggregation accumulator, see$first (aggregation accumulator)
.
Syntax
$first
has the following syntax:
{ $first: <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 $first
operator is an alias for the following
$arrayElemAt
expression:
{ $arrayElemAt: [ <array expression>, 0 ] }
Behavior
Valid Operands
Valid operand for $first
must resolve to an array,
null, or missing
If the operand resolves to a non-empty array,
$first
returns the first element in the array:If the operand resolves to an empty array
[]
,$first
does not return a value.If the operand is null or missing,
$first
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 firstElem
whose value is
derived from applying the $first
operator to the x
field:
db.example1.aggregate([ { $addFields: { firstElem: { $first: "$x" } } } ])
The operator returns the following documents:
{ "_id" : 1, "x" : [ 1, 2, 3 ], "firstElem" : 1 } { "_id" : 2, "x" : [ [ ] ], "firstElem" : [ ] } { "_id" : 3, "x" : [ null ], "firstElem" : null } { "_id" : 4, "x" : [ ] } // No output { "_id" : 5, "x" : null, "firstElem" : null } { "_id" : 6, "firstElem" : 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: { firstElem: { $first: "$x" } } } )
That is, the operation returns the following:
2020-01-20T18:31:13.431-05:00 E QUERY [js] uncaught exception: Error: command failed: { "ok" : 0, "errmsg" : "$first'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." }