Introduction to MongoDB $concatArrays Operator

In MongoDB’s aggregation pipeline, the $concatArrays operator is used to merge two or more arrays into one array.

Syntax

The syntax of the $concatArrays operator is as follows:

{  `$concatArrays` : [ <expression1>, <expression2>, ... ] }

Where <expression1>, <expression2> are the array expressions to be merged.

Use Cases

The $concatArrays operator is suitable for aggregation operations that require merging multiple arrays. For example, it can merge arrays from different data sources into one array.

Examples

Example 1

The following aggregation pipeline merges data from the sales and expenses collections into one array:

db.sales.aggregate([
   {
      $lookup:
         {
           from: "expenses",
           localField: "quarter",
           foreignField: "quarter",
           as: "expenses"
         }
   },
   {
      $project:
         {
           _id: 0,
           quarter: 1,
           sales: 1,
           expenses: "$expenses.amount"
         }
   },
   {
      $project:
         {
           _id: 0,
           data: {  `$concatArrays` : [ "$sales", "$expenses" ] }
         }
   }
])

After running the above aggregation pipeline, the result is as follows:

{ "data" : [ 100, 200, 300, 50, 75, 100 ] }

Example 2

The following aggregation pipeline uses the $concatArrays operator to merge two arrays in a document into one array:

db.collection.aggregate([
   {
      $project:
         {
            _id: 0,
            merged: {  `$concatArrays` : [ "$array1", "$array2" ] }
         }
   }
])

Suppose the original document is as follows:

{ "array1" : [ 1, 2, 3 ], "array2" : [ 4, 5, 6 ] }