$slice (projection)

$slice of Embedded Array

Starting in MongoDB 4.4, the $slice projection of an array in an nested document no longer returns the other fields in the nested document when the projection is part of an inclusion projection.

For example, consider a collection inventory with documents that contain a size field:

{ item: "socks", qty: 100, details: { colors: [ "blue", "red" ], sizes: [ "S", "M", "L"] } }

Starting in MongoDB 4.4, the following operation projects the _id field (by default), the qty field, and the details field with just the specified slice of the colors array:

db.inventory.find( { }, { qty: 1, "details.colors": { $slice: 1 } } )

That is, the operation returns the following document:

{ "_id" : ObjectId("5ee92a6ec644acb6d13eedb1"), "qty" : 100, "details" : { "colors" : [ "blue" ] } }

If the $slice projection is part of an exclusion projection, the operation continues to return the other fields in the nested document. That is, the following projection is an exclusion projection. The projection excludes the _id field and the elements in the colors array that fall outside the specified slice and returns all other fields.

db.inventory.find( { }, { _id: 0, "details.colors": { $slice: 1 } } )

{ "item" : "socks", "qty" : 100, "details" : { "colors" : [ "blue" ], "sizes" : [ "S", "M", "L" ] } }

The $slice projection by itself is considered an exclusion.

In previous versions, the $slice projection also include the other fields in the nested document regardless of whether the projection is an inclusion or an exclusion.

View Restriction

db.collection.find() operations on views do not support $slice projection operator.

$ Positional Operator and $slice Restriction

Starting in MongoDB 4.4, find and findAndModify projection cannot include $slice projection expression as part of a $ projection expression.

For example, starting in MongoDB 4.4, the following operation is invalid:

db.inventory.find( { "instock.qty": { $gt: 25 } }, { "instock.$": { $slice: 1 } } ) // Invalid starting in 4.4

MongoDB already has a restriction where top-level field names cannot start with the dollar sign ($).

In previous versions, MongoDB returns the first element (instock.$) in the instock array that matches the query condition; i.e. the positional projection "instock.$" takes precedence and the $slice:1 is a no-op. The "instock.$": { $slice: 1 } does not exclude any other document field.

Path Collision: $slice of an Array and Embedded Fields

Starting in MongoDB 4.4, find and findAndModify projection cannot contain both a $slice of an array and a field embedded in the array.

For example, consider a collection inventory that contains an array field instock:

{ ..., instock: [ { warehouse: "A", qty: 35 }, { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ], ... }

Starting in MongoDB 4.4, the following operation fails with a Path collision error:

db.inventory.find( {}, { "instock": { $slice: 1 }, "instock.warehouse": 0 } ) // Invalid starting in 4.4

In previous versions, the projection applies both projections and returns the first element ($slice: 1) in the instock array but suppresses the warehouse field in the projected element. Starting in MongoDB 4.4, to achieve the same result, use the db.collection.aggregate() method with two separate $project stages.

Return an Array with Its First 3 Elements

The following operation uses the $slice projection operator on the comments array to return the array with its first three elements. If the array has less than three elements, all elements in the array are returned.

db.posts.find( {}, { comments: { $slice: 3 } } )

The operation returns the following documents:

{
   "_id" : 1,
   "title" : "Bagels are not croissants.",
   "comments" : [ { "comment" : "0. true" }, { "comment" : "1. croissants aren't bagels." } ]
}
{
   "_id" : 2,
   "title" : "Coffee please.",
   "comments" : [ { "comment" : "0. fooey" }, { "comment" : "1. tea please" }, { "comment" : "2. iced coffee" } ]
}

Return an Array with Its Last 3 Elements

The following operation uses the $slice projection operator on the comments array to return the array with its last three elements. If the array has less than three elements, all elements in the array are returned.

db.posts.find( {}, { comments: { $slice: -3 } } )

The operation returns the following documents:

{
   "_id" : 1,
   "title" : "Bagels are not croissants.",
   "comments" : [ { "comment" : "0. true" }, { "comment" : "1. croissants aren't bagels." } ]
}
{
   "_id" : 2,
   "title" : "Coffee please.",
   "comments" : [ { "comment" : "2. iced coffee" }, { "comment" : "3. cappuccino" }, { "comment" : "4. whatever" } ]
}

Return an Array with 3 Elements After Skipping the First Element

The following operation uses the $slice projection operator on the comments array to:

• Skip the first element such that the second element is the starting point.

• Then, return three elements from the starting point.

If the array has less than three elements after the skip, all remaining elements are returned.

db.posts.find( {}, { comments: { $slice: [ 1, 3 ] } } )

The operation returns the following documents:

{
   "_id" : 1,
   "title" : "Bagels are not croissants.",
   "comments" : [ { "comment" : "1. croissants aren't bagels." } ]
}
{
   "_id" : 2,
   "title" : "Coffee please.",
   "comments" : [ { "comment" : "1. tea please" }, { "comment" : "2. iced coffee" }, { "comment" : "3. cappuccino" } ]
}

Return an Array with 3 Elements After Skipping the Last Element

The following operation uses the $slice projection operator on the comments array to

• Skip backwards from the first element such that the last element is the starting point.

• Then, return three elements from the starting point.

If the array has less than three elements after the skip, all remaining elements in the array are returned.

db.posts.find( {}, { comments: { $slice: [ -1, 3 ] } } )

The operation returns the following documents:

{
   "_id" : 1,
   "title" : "Bagels are not croissants.",
   "comments" : [ { "comment" : "1. croissants aren't bagels." } ]
}
{
   "_id" : 2,
   "title" : "Coffee please.",
   "comments" : [ { "comment" : "4. whatever" } ]
}