MongoDB - $concat Operator

The $concat operator in MongoDB concatenates multiple strings or expressions into a single string within the aggregation pipeline, enabling efficient string formatting and reporting during data transformation.

• Commonly used in aggregation stages like $project, $addFields, and others for string transformations.
• Accepts multiple string expressions and computed expressions as inputs.
• Supports combining field values, literals, and expression results.
• Enables complex string formatting for reporting and presentation.
• Evaluates expressions server-side for efficient transformations.

Syntax

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

• Accepts multiple arguments: string literals, fields, or expressions that resolve to strings.
• Each input expression is evaluated before concatenation.
• Produces a single combined string as the output.

Features of $concat Operator

Some features are listed below:

• Input Expressions : Accepts multiple arguments, including string literals, field references, and expressions that resolve to strings.
• Null Handling : Returns null if any argument evaluates to null.
• Missing Fields : Returns null if any referenced field is missing in the document.
• Output Type : Produces a single concatenated string as the result.
• Usage Context : Commonly used in the $project stage of aggregation pipelines for string formatting.

Use Cases for MongoDB $concat Operator

Here are a few practical scenarios where the $concat operator shines:

• Concatenating First and Last Names: Combine the first name, middle name, and last name to create a full name for a user.
• Adding Static Prefixes or Suffixes: Prefix or suffix text to data, such as adding "Hello, " before a name or " - Employee" after a title.
• Building Custom Messages: Create personalized messages, for example, for a report or a notification.

Examples of MongoDB $concat Operator

We'll use a sample collection called employee in the GeeksforGeeks database, which contains employee information.

• Database: GeeksforGeeks
• Collection: employee
• Document: Three documents that contain the details of the employees in the form of field-value pairs.

Example 1: Concatenating Strings Using $concat

Formatting a string by combining a static message with the department field. Here, we concatenate "My department is:" string with the value of the department field.

db.employee.aggregate([
 {
 $project: {
 "name.first": 1,
 _id: 0,
 dept: { $concat: ["My department is: ", "$department"] }
 }
 }
])

Output:

• Projects only the employee’s first name.
• Concatenates a custom string with the department field.
• Creates a formatted message indicating the employee’s department.

Example 2: Concatenating Strings in Embedded Documents

Find the full name of the employees by concatenating the values of the name.first, name.middle, and name.last fields. 

db.employee.aggregate([
 {
 $project: {
 fullname: {
 $concat: ["$name.first", " ", "$name.middle", " ", "$name.last"]
 }
 }
 }
])

Output:

• Concatenates first, middle, and last name fields into fullname.
• Creates a new field fullname in the output.
• Provides the employee’s complete name in a single string.

Usage of $concat Operator

Here are some usage of $concat operator:

• Aggregation Usage: Commonly used in the $project stage to create new computed fields.
• Flexible Inputs: Accepts field references, string literals, and computed expressions.
• Null Handling: Returns null if any argument is null or a referenced field is missing.
• Composable: Works with operators like $trim, $toUpper, and $toLower for advanced formatting.
• Multiple Fields: Merges values from multiple fields into a single string.
• Dynamic Strings: Builds labels, messages, or formatted outputs at query time.
• Data Transformation: Adds prefixes/suffixes and formats strings for reporting or UI output.