Database Manual / Reference / Query Language / Expressions

$setEquals (aggregation)

Definition

$setEquals

Compares two or more arrays and returns true if they have the same distinct elements and false otherwise.

$setEquals has the following syntax:

{ $setEquals: [ <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

$setEquals performs set operation on arrays, treating arrays as sets. If an array contains duplicate entries, $setEquals ignores the duplicate entries. $setEquals ignores the order of the elements.

If a set contains a nested array element, $setEquals does not descend into the nested array but evaluates the array at top-level.

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

false

The arrays do not contain the same elements.

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

false

The arrays do not contain the same elements.

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

true

Both arrays contain the same unique elements. When treated as sets, the first array is [ "a", "b" ] because duplicates are ignored.

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

false

The elements in the arrays are not equal. The first array contains the strings "a" and "b", while the second contains an array [ "a", "b" ].

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

false

The elements in the arrays are not equal. The first array is empty and the second array has two string elements.

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

false

The arrays do not contain the same unique elements. The first array only contains "a", while the second array contains "a" and "b".

Example

Consider a bakeryOrders collection with the following documents:

db.bakeryOrders.insertMany( [
   { _id: 0, cakes: ["chocolate", "vanilla"], cupcakes: ["chocolate", "vanilla"] },
   { _id: 1, cakes: ["chocolate", "vanilla"], cupcakes: ["vanilla", "chocolate"] },
   { _id: 2, cakes: ["chocolate", "chocolate"], cupcakes: ["chocolate"] },
   { _id: 3, cakes: ["vanilla"], cupcakes: ["chocolate"] },
   { _id: 4, cakes: ["vanilla"], cupcakes: [] }
] )

The following operation uses the $setEquals operator to determine if the cakes array and the cupcakes array in each order contain the same flavors:

db.bakeryOrders.aggregate(
   [
      {
         $project: {
            _id: 0,
            cakes: 1,
            cupcakes: 1,
            sameFlavors: { $setEquals: [ "$cakes", "$cupcakes" ] }
         }
      }
   ] )

Note

$project

The $project stage specifies which fields are included in the output documents. In this example, the $project stage:

  • Excludes the _id field from the output.
  • Includes the cakes and cupcakes fields in the output.
  • Outputs the result of the $setEquals operator in a new field called sameFlavors.

The operation returns the following results:

{
 cakes: [ "chocolate", "vanilla" ],
 cupcakes: [ "chocolate", "vanilla" ],
 sameFlavors: true
},
{
 cakes: [ "chocolate", "vanilla" ],
 cupcakes: [ "vanilla", "chocolate" ],
 sameFlavors: true
},
{
 cakes: [ "chocolate", "chocolate" ],
 cupcakes: [ "chocolate" ],
 sameFlavors: true
},
{
 cakes: [ "vanilla" ],
 cupcakes: [ "chocolate" ],
 sameFlavors: false
},
{
   cakes: [ "vanilla" ],
   cupcakes: [],
   sameFlavors: false
}