MongoDB 4.4 introduced the $first aggregation pipeline operator.

This operator returns the first element in an array.

Example

Suppose we have a collection called players with the following documents:

{ "_id" : 1, "player" : "Homer", "scores" : [ 1, 5, 3 ] }
{ "_id" : 2, "player" : "Marge", "scores" : [ 8, 17, 18 ] }
{ "_id" : 3, "player" : "Bart", "scores" : [ 15, 11, 8 ] }

We can see that each document has a scores field that contains an array.

We can use $first to return the first element of each of those arrays.

Example:

db.players.aggregate([ 
  {
    $project: {
        "firstScore": { 
          $first: "$scores" 
        }
    }
  } 
])

Result:

{ "_id" : 1, "firstScore" : 1 }
{ "_id" : 2, "firstScore" : 8 }
{ "_id" : 3, "firstScore" : 15 }

We can see that the first element of the array was returned for each document.

This is the equivalent of using the $arrayElemAt operator with a value of zero (0):

db.players.aggregate([
  {
    $project: {
      "firstScore": { $arrayElemAt: [ "$scores", 0 ] }
    }
  }
])

Empty Arrays

If you provide an empty array, $first will not return a value.

Suppose we insert the following document into our collection:

{ "_id" : 4, "player" : "Farnsworth", "scores" : [ ] }

Let’s run the code again:

db.players.aggregate([ 
  {
    $project: {
        "firstScore": { 
          $first: "$scores" 
        }
    }
  } 
])

Result:

{ "_id" : 1, "firstScore" : 1 }
{ "_id" : 2, "firstScore" : 8 }
{ "_id" : 3, "firstScore" : 15 }
{ "_id" : 4 }

In this case, document 4 didn’t return any value for the array. In fact, it didn’t even return the field name.

Null and Missing Values

If the operand is null or missing, $first returns null.

Suppose we insert the following document:

{ "_id" : 5, "player" : "Meg", "scores" : null }

Let’s run the code again:

db.players.aggregate([ 
  {
    $project: {
        "firstScore": { 
          $first: "$scores" 
        }
    }
  } 
])

Result:

{ "_id" : 1, "firstScore" : 1 }
{ "_id" : 2, "firstScore" : 8 }
{ "_id" : 3, "firstScore" : 15 }
{ "_id" : 4 }
{ "_id" : 5, "firstScore" : null }

This time it returned the field with a value of null.

Invalid Operand

The operand for $first must resolve to an array, null, or missing. Providing an invalid operand results in an error.

To demonstrate this, let’s try to use $first against the player field (which isn’t an array):

db.players.aggregate([ 
  {
    $project: {
        "firstPlayer": { 
          $first: "$player" 
        }
    }
  } 
])

Result:

Error: command failed: {
	"ok" : 0,
	"errmsg" : "$first's argument must be an array, but is string",
	"code" : 28689,
	"codeName" : "Location28689"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:618:17
assert.commandWorked@src/mongo/shell/assert.js:708:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1046:12
@(shell):1:1

As expected, it returned an error.