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- $slice (aggregation)
$slice (aggregation)¶
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Definition¶
- $slice¶
3.2 新版功能.
Returns a subset of an array.
$slice has one of two syntax forms:
The following syntax returns elements from either the start or end of the array:
{ $slice: [ <array>, <n> ] }
The following syntax returns elements from the specified position in the array:
{ $slice: [ <array>, <position>, <n> ] }
Operand Description <array> Any valid expression as long as it resolves to an array. <position> Optional. Any valid expression as long as it resolves to an integer.
- If positive, $slice determines the starting position from the start of the array. If <position> is greater than the number of elements, the $slice returns an empty array.
- If negative, $slice determines the starting position from the end of the array. If the absolute value of the <position> is greater than the number of elements, the starting position is the start of the array.
<n> Any valid expression as long as it resolves to an integer. If <position> is specified, <n> must resolve to a positive integer.
- If positive, $slice returns up to the first n elements in the array. If the <position> is specified, $slice returns the first n elements starting from the position.
- If negative, $slice returns up to the last n elements in the array. n cannot resolve to a negative number if <position> is specified.
For more information on expressions, see Expressions.
Behavior¶
| Example | Results |
|---|---|
| { $slice: [ [ 1, 2, 3 ], 1, 1 ] } | [ 2 ] |
| { $slice: [ [ 1, 2, 3 ], -2 ] } | [ 2, 3 ] |
| { $slice: [ [ 1, 2, 3 ], 15, 2 ] } | [ ] |
| { $slice: [ [ 1, 2, 3 ], -15, 2 ] } | [ 1, 2 ] |
Example¶
A collection named users contains the following documents:
{ "_id" : 1, "name" : "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] }
{ "_id" : 2, "name" : "li", favorites: [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", favorites: [ "pears", "pecans", "chocolate", "cherries" ] }
{ "_id" : 4, "name" : "ty", favorites: [ "ice cream" ] }
The following example returns at most the first three elements in the favorites array for each user:
db.users.aggregate([
{ $project: { name: 1, threeFavorites: { $slice: [ "$favorites", 3 ] } } }
])
The operation returns the following results:
{ "_id" : 1, "name" : "dave123", "threeFavorites" : [ "chocolate", "cake", "butter" ] }
{ "_id" : 2, "name" : "li", "threeFavorites" : [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", "threeFavorites" : [ "pears", "pecans", "chocolate" ] }
{ "_id" : 4, "name" : "ty", "threeFavorites" : [ "ice cream" ] }