MongoDB $concatArrays Operator

MongoDB provides different types of array expression operators that are used in the aggregation pipeline stages and $concatArrays operator is one of them. This operator is used to concatenate two or more arrays and return a concatenated array.

Syntax: 

{ $concatArrays: [ <array1>, <array2>, ... <arrayN>] }

Here, the array must be a valid expression until it resolves to an array. If the argument of this operator is a missing field or the argument resolve to a null value, then this operator will give null.

Examples:

In the following examples, we are working with:

Database: GeeksforGeeks

Collection: arrayExample

Document: three documents that contain the details in the form of field-value pairs.

Using $concatArrays Operator:

In this example, we are going to concatenate the values(i.e., arrays) of  numbers1 and numbers2 fields using $concatArrays operator.

db.arrayExample.aggregate([
... {$match: {name: "Lolo"}},
... {$project: {
... concatResult: {$concatArrays: ["$numbers1", "$numbers2"]}}}])

In this example, we are going to concatenate the values(i.e., arrays) of fruits and veggie fields using $concatArrays operator.

db.arrayExample.aggregate([
... {$match: {name: "Bongo"}},
... {$project: {concatResult:
... {$concatArrays: ["$fruits", "$veggie"]}}}])

Using $concatArrays Operator in the Embedded Document:

In this example, we are going to concatenate the value(i.e., array) of favGame.indoorGames field with the specified array(i.e., [“Uno”, “Snooker”]) using $concatArrays operator.

db.arrayExample.aggregate([{$match: {name: "Piku"}},
... {$project: {concatResult: {
... $concatArrays: ["$favGame.indoorGames", ["Uno", "Snooker"]]}}}])

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