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$setIsSubset (aggregation)

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  • Definition
  • Behavior
  • Example
$setIsSubset

Takes two arrays and returns true when the first array is a subset of the second, including when the first array equals the second array, and false otherwise.

$setIsSubset has 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 Expression Operators.

$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
{ $setIsSubset: [ [ "a", "b", "a" ], [ "b", "a" ] ] }
true
{ $setIsSubset: [ [ "a", "b" ], [ [ "a", "b" ] ] ] }
false

Consider an flowers collection with the following documents:

db.flowers.insertMany( [
{ "_id" : 1, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid" ] },
{ "_id" : 2, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "orchid", "rose", "orchid" ] },
{ "_id" : 3, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid", "jasmine" ] },
{ "_id" : 4, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "jasmine", "rose" ] },
{ "_id" : 5, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ ] },
{ "_id" : 6, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose" ], [ "orchid" ] ] },
{ "_id" : 7, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose", "orchid" ] ] },
{ "_id" : 8, "flowerFieldA" : [ ], "flowerFieldB" : [ ] },
{ "_id" : 9, "flowerFieldA" : [ ], "flowerFieldB" : [ "rose" ] }
] )

The following operation uses the $setIsSubset operator to determine if the flowerFieldA array is a subset of the flowerFieldB array:

db.flowers.aggregate(
[
{ $project: { flowerFieldA:1, flowerFieldB: 1, AisSubset: { $setIsSubset: [ "$flowerFieldA", "$flowerFieldB" ] }, _id:0 } }
]
)

The operation returns the following results:

{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid" ], "AisSubset" : true }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "orchid", "rose", "orchid" ], "AisSubset" : true }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid", "jasmine" ], "AisSubset" : true }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "jasmine", "rose" ], "AisSubset" : false }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ ], "AisSubset" : false }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose" ], [ "orchid" ] ], "AisSubset" : false }
{ "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose", "orchid" ] ], "AisSubset" : false }
{ "flowerFieldA" : [ ], "flowerFieldB" : [ ], "AisSubset" : true }
{ "flowerFieldA" : [ ], "flowerFieldB" : [ "rose" ], "AisSubset" : true }
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