MongoDB $concatArrays Operator (original) (raw)

Last Updated : 5 May, 2026

The $concatArrays operator in MongoDB combines two or more arrays into a single array within the aggregation pipeline, preserving the original order of elements.

• Merges multiple array fields into one unified array.
• Preserves the order of elements from input arrays.
• Works within aggregation stages for data transformation.
• Can combine array fields with static arrays or expressions.

Syntax

{ $concatArrays: [ , , ... ] }

• ****, , ..., :** These are the arrays to be concatenated. Each of these arrays must be a valid expression that resolves to an array.
• The operator will return a single array that contains the elements of all input arrays, in the order provided.
• If any of the input arrays are null or missing, the operator will return null.

Features of the $concatArrays Operator

Here are some features of $concatArrays operator:

• **Concatenation of Multiple Arrays: The operator can merge any number of arrays, regardless of their sizes or data types.
• **Array Preservation: It preserves the order of elements from the original arrays.
• **Handling Missing Arrays: When arrays are missing or null, the operator returns null, making it essential to handle these cases with conditional logic.
• **Versatility: It works well with both arrays stored in different fields and static arrays defined inline.

Examples of MongoDB $concatArrays

Let's go through some practical examples to understand how the $concatArrays operator works. 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__._

Example 1: Using $concatArrays Operator

Concatenate the values (i.e., arrays) of numbers1 and numbers2 fields using $concatArrays operator.

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

**Output:

• numbers1 and numbers2 are merged into one array using $concatArrays.
• The result preserves order: [2, 4, 5, 6, 7, 8, 29, 2, 4, 5, 6].

Example 2: Using $concatArrays Operator to concatenate arrays Values

Concatenate the fruits and veggie arrays into a single array for a document with name: "Bruno" using $concatArrays operator.

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

**Output:

• $concatArrays merges the fruits and veggie arrays into a single array.
• The combined result preserves order: ["Mango", "Banana", "Peach", "Apple", "Broccoli", "Tomato", "Pumpkin"].

Example 3: Using $concatArrays Operator in the Embedded Document

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: "Pika" } },
{
$project: {
concatResult: {
concatArrays:["concatArrays: ["concatArrays:["favGame.indoorGames", ["Uno", "Snooker"]]
}
}
}
])

**Output:

• Appends ["Uno", "Snooker"] to the existing favGame.indoorGames array.
• Result becomes ["Ludo", "Chess", "Uno", "Snooker"].

Use Cases for MongoDB $concatArrays Operator

The $concatArrays operator is used in various scenarios, including:

• **Combining Data from Multiple Fields: Merging arrays from different fields in a single document.
• **Concatenating Arrays Across Different Documents: Aggregating arrays from different documents into one.
• **Adding Static Arrays to Existing Data: Concatenating predefined static arrays with arrays from your documents.
• **Flattening Data: Helps combine arrays, but true flattening of nested arrays requires operators like reduceorreduce or reduceorunwind.