MongoDB $concatArrays Operator

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: [ <array1>, <array2>, ... <arrayN> ] }

• <array1>, <array2>, ..., <arrayN>: 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: { $concatArrays: ["$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: { $concatArrays: ["$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: {
        $concatArrays: ["$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 $reduce or $unwind.