MongoDB – $concat Operator

MongoDB $concat or concatenation operator concatenates two or more strings and returns a single string. The $concat operator is one of the string expression operators that is used in the aggregation pipeline stages.

Syntax

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

Here, the arguments passed in this operator can be any valid expression until they resolve to strings.

• If the entered argument resolves to null, then this operator will return null.
• If the entered argument refers to a missing field, then this operator will return null.

MongoDB $concat Operator Examples

In the following examples, we are working with:

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 operator

In this example, we are finding the department of the employees. Here, we concatenate “My department is:” string with the value of the department field.

Query:

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

Output:

Example 2: Concatenating strings in the embedded documents using $concat operator

In this example, we are going to find the full name of the employees by concatenating the values of the name.first, name.middle, and name.last fields. 

Query:

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

Output:

Key Takeaways About $concat Operator

• The $concat operator in MongoDB is used to concatenate two or more strings into a single string.
• It is a powerful tool for string manipulation in MongoDB.
• It can be used with the $project stage of the aggregation pipeline to add a new field to the documents with the concatenated string.
• The $concat operator can be used with other aggregation operators, such as $trim, to further manipulate the strings.