$setIsSubset (aggregation)
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Definition
$setIsSubset
Takes two arrays and returns
true
when the first array is a subset of the second, including when the first array equals the second array, andfalse
otherwise.$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.
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 }