$setIsSubset (aggregation)
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Definition
$setIsSubset-
New in version 2.6.
Takes two arrays and returns
truewhen the first array is a subset of the second, including when the first array equals the second array, andfalseotherwise.$setIsSubsethas the following syntax:{ $setIsSubset: [ <expression1>, <expression2> ] }The arguments can be any valid expression as long as they each resolve to an array. For more information on expressions, see Expressions.
Behavior
$setIsSubset performs set operation on arrays, treating arrays as sets. If an array contains duplicate entries, $setIsSubset ignores the duplicate entries. $setIsSubset ignores the order of the elements.
If a set contains a nested array element, $setIsSubset does not descend into the nested array but evaluates the array at top-level.
| Example | Result |
|---|---|
|
true |
|
false |
Example
Consider an experiments collection with the following documents:
{ "_id" : 1, "A" : [ "red", "blue" ], "B" : [ "red", "blue" ] }
{ "_id" : 2, "A" : [ "red", "blue" ], "B" : [ "blue", "red", "blue" ] }
{ "_id" : 3, "A" : [ "red", "blue" ], "B" : [ "red", "blue", "green" ] }
{ "_id" : 4, "A" : [ "red", "blue" ], "B" : [ "green", "red" ] }
{ "_id" : 5, "A" : [ "red", "blue" ], "B" : [ ] }
{ "_id" : 6, "A" : [ "red", "blue" ], "B" : [ [ "red" ], [ "blue" ] ] }
{ "_id" : 7, "A" : [ "red", "blue" ], "B" : [ [ "red", "blue" ] ] }
{ "_id" : 8, "A" : [ ], "B" : [ ] }
{ "_id" : 9, "A" : [ ], "B" : [ "red" ] }
The following operation uses the $setIsSubset operator to determine if the A array is a subset of the B array:
db.experiments.aggregate(
[
{ $project: { A:1, B: 1, AisSubset: { $setIsSubset: [ "$A", "$B" ] }, _id:0 } }
]
)
The operation returns the following results:
{ "A" : [ "red", "blue" ], "B" : [ "red", "blue" ], "AisSubset" : true }
{ "A" : [ "red", "blue" ], "B" : [ "blue", "red", "blue" ], "AisSubset" : true }
{ "A" : [ "red", "blue" ], "B" : [ "red", "blue", "green" ], "AisSubset" : true }
{ "A" : [ "red", "blue" ], "B" : [ "green", "red" ], "AisSubset" : false }
{ "A" : [ "red", "blue" ], "B" : [ ], "AisSubset" : false }
{ "A" : [ "red", "blue" ], "B" : [ [ "red" ], [ "blue" ] ], "AisSubset" : false }
{ "A" : [ "red", "blue" ], "B" : [ [ "red", "blue" ] ], "AisSubset" : false }
{ "A" : [ ], "B" : [ ], "AisSubset" : true }
{ "A" : [ ], "B" : [ "red" ], "AisSubset" : true }