$isArray (aggregation)
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Definition
$isArrayDetermines if the operand is an array. Returns a boolean.
$isArrayhas the following syntax:{ $isArray: [ <expression> ] }
Behavior
The <expression> can be any valid expression. For more information on expressions, see
Expressions.
Example | Results | Notes |
|---|---|---|
{ $isArray: "hello" } | false | "hello" is a string, passed as a string. |
{ $isArray: [ "hello" ] } | false | "hello" is a string, passed as part of an argument array. |
{ $isArray: [ [ "hello" ] ] } | true | [ "hello" ] is an array, passed as part of an argument array. |
Note
Aggregation expressions accept a variable number of arguments. These arguments are normally passed as an array. However, when the argument is a single value, you can simplify your code by passing the argument directly without wrapping it in an array.
Example
Create the warehouses collection:
db.warehouses.insertMany( [ { "_id" : 1, instock: [ "chocolate" ], ordered: [ "butter", "apples" ] }, { "_id" : 2, instock: [ "apples", "pudding", "pie" ] }, { "_id" : 3, instock: [ "pears", "pecans"], ordered: [ "cherries" ] }, { "_id" : 4, instock: [ "ice cream" ], ordered: [ ] } ] )
Check if the instock and the ordered fields are arrays. If both
fields are arrays, concatenate them:
db.warehouses.aggregate( [ { $project: { items: { $cond: { if: { $and: [ { $isArray: "$instock" }, { $isArray: "$ordered" } ] }, then: { $concatArrays: [ "$instock", "$ordered" ] }, else: "One or more fields is not an array." } } } } ] )
{ "_id" : 1, "items" : [ "chocolate", "butter", "apples" ] } { "_id" : 2, "items" : "One or more fields is not an array." } { "_id" : 3, "items" : [ "pears", "pecans", "cherries" ] } { "_id" : 4, "items" : [ "ice cream" ] }