Database Manual / Reference / Query Language / Expressions

$setIsSubset (aggregation)

Definition

$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 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.

ExampleResultNotes
 
{ $setIsSubset: [ [ "a", "c" ], [ "a", "b" ] ] }

false

Not a subset, because c is not present in the second array.

{ $setIsSubset: [ [ "a", "c" ], [ "a", "b", "c" ] ] }

true

Both elements in the first array are present in the second array.

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

true

Only the unique elements of the first array ["a", "b"] are considered, so the first array is a subset of the second and the result is true, regardless of duplicate elements or their order.

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

false

The second array [ [ "a", "b" ] ] contains a single element, wich itself is an array. Therefore, the first array [ "a", "b" ] is not considered a subset of the second and the result is false.

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

true

An empty set is considered a subset of any set.

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

true

A set is a subset of itself. Duplicate elements do not affect subset evaluation.

Example

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 }