String expressions ($concat, $toUpper, $toLower) in MongoDB - Mini Project: Build & Apply

Using String Expressions in MongoDB Aggregation

📖 Scenario: You work in a small bookstore's database team. The store keeps records of books with separate fields for the author's first name and last name. You want to create a new field that shows the author's full name in uppercase letters for better display on the website.

🎯 Goal: Build a MongoDB aggregation pipeline that combines the author's first and last names into a single full name string, converts it to uppercase, and stores it in a new field called authorFullName.

📋 What You'll Learn

Create a collection named books with documents containing authorFirstName and authorLastName fields.

Add a configuration variable space that holds a single space character.

Use the $concat expression to join the first name, space, and last name into one string.

Use the $toUpper expression to convert the concatenated full name to uppercase.

Add the resulting uppercase full name as a new field authorFullName in the aggregation output.

💡 Why This Matters

🌍 Real World

Bookstores and libraries often store author names in separate fields. Combining and formatting these names helps display them nicely on websites and reports.

💼 Career

Database developers and data engineers frequently use string expressions in MongoDB aggregation pipelines to transform and prepare data for applications.

1

Create the books collection with author names

Create a MongoDB collection called books with these exact documents: { authorFirstName: "Jane", authorLastName: "Austen" } and { authorFirstName: "Mark", authorLastName: "Twain" }.

MongoDB

// Insert the two book documents into the books collection
// Your code here

Hint

Use insertMany to add multiple documents to the books collection.

2

Add a space variable for joining names

Create a variable called space and set it to a string containing a single space character " ".

MongoDB

db.books.insertMany([
  { authorFirstName: "Jane", authorLastName: "Austen" },
  { authorFirstName: "Mark", authorLastName: "Twain" }
])

// Create a variable called space with a single space string
// Your code here

Hint

Use const space = " " to store a space character.

3

Use $concat to join first and last names with space

Write a MongoDB aggregation pipeline stage that uses $concat to join authorFirstName, the variable space, and authorLastName into a new field called fullName.

MongoDB

db.books.insertMany([
  { authorFirstName: "Jane", authorLastName: "Austen" },
  { authorFirstName: "Mark", authorLastName: "Twain" }
])

const space = " "

// Create an aggregation pipeline stage that adds fullName by concatenating authorFirstName, space, and authorLastName
// Your code here

Hint

Use $addFields with $concat to join the fields and the space variable.

4

Convert the full name to uppercase with $toUpper

Extend the aggregation pipeline to add a new field called authorFullName that converts the fullName field to uppercase using $toUpper.

MongoDB

db.books.insertMany([
  { authorFirstName: "Jane", authorLastName: "Austen" },
  { authorFirstName: "Mark", authorLastName: "Twain" }
])

const space = " "

const pipeline = [
  {
    $addFields: {
      fullName: { $concat: ["$authorFirstName", space, "$authorLastName"] }
    }
  }
]

// Add authorFullName field that converts fullName to uppercase
// Your code here

Hint

Use $toUpper around the fullName field to create authorFullName.

Practice

1. What does the MongoDB expression {$toUpper: "hello"} do?

easy

A. Joins the string "hello" with another string

B. Converts the string "hello" to lowercase "hello"

C. Converts the string "hello" to uppercase "HELLO"

D. Removes spaces from the string "hello"

Solution

1. Step 1: Understand $toUpper usage

   The $toUpper operator converts all letters in a string to uppercase.

2. Step 2: Apply $toUpper to "hello" string

   Applying $toUpper to "hello" changes it to "HELLO".

3. Final Answer:

   Converts the string "hello" to uppercase "HELLO" -> Option C

4. Quick Check:

   $toUpper("hello") = "HELLO" [OK]

Hint: Remember $toUpper makes all letters uppercase [OK]

Common Mistakes:

• Confusing $toUpper with $toLower
• Thinking $toUpper joins strings
• Assuming it removes spaces

2. Which of the following is the correct syntax to concatenate two fields firstName and lastName in MongoDB aggregation?

easy

A. {$concat: "$firstName" + " " + "$lastName"}

B. {$concat: ["$firstName", " ", "$lastName"]}

C. {$concat: "$firstName", " ", "$lastName"}

D. {$concat: "$firstName $lastName"}

Solution

1. Step 1: Recall $concat syntax

   $concat requires an array of strings or field references to join them.

2. Step 2: Check each option's syntax

   {$concat: ["$firstName", " ", "$lastName"]} correctly uses an array with field references and a space string. Others misuse concatenation or array syntax.

3. Final Answer:

   {$concat: ["$firstName", " ", "$lastName"]} -> Option B

4. Quick Check:

   $concat needs array of strings [OK]

Hint: Use array syntax with $concat for multiple strings [OK]

Common Mistakes:

• Using + operator inside $concat
• Passing arguments without array brackets
• Putting fields inside a single string

3. Given the document {"name": "alice", "surname": "smith"}, what is the output of this aggregation expression?

{ $concat: [ { $toUpper: "$name" }, " ", { $toLower: "$surname" } ] }

medium

A. "Alice Smith"

B. "alice smith"

C. "ALICE SMITH"

D. "ALICE smith"

Solution

1. Step 1: Apply $toUpper to "name" field

   The "name" field is "alice", $toUpper converts it to "ALICE".

2. Step 2: Apply $toLower to "surname" field and concatenate

   The "surname" field is "smith", $toLower keeps it "smith". Then $concat joins "ALICE", space, and "smith".

3. Final Answer:

   "ALICE smith" -> Option D

4. Quick Check:

   $toUpper("alice") + " " + $toLower("smith") = "ALICE smith" [OK]

Hint: Apply $toUpper and $toLower before concatenation [OK]

Common Mistakes:

• Assuming $toLower changes uppercase to title case
• Mixing order of operations
• Forgetting to use $concat array

4. Identify the error in this MongoDB aggregation expression:

{ $concat: "$firstName", " ", "$lastName" }

medium

A. Missing array brackets around the arguments for $concat

B. Using double quotes instead of single quotes

C. Incorrect field names without $ prefix

D. Using $concat instead of $toUpper

Solution

1. Step 1: Check $concat argument format

   $concat requires a single array argument containing strings or field references.

2. Step 2: Identify missing array brackets

   The expression passes multiple arguments without wrapping them in an array, causing syntax error.

3. Final Answer:

   Missing array brackets around the arguments for $concat -> Option A

4. Quick Check:

   $concat([...]) needs array brackets [OK]

Hint: Always wrap $concat inputs in an array [OK]

Common Mistakes:

• Passing multiple arguments without array
• Confusing $concat with string concatenation operator
• Not prefixing field names with $

5. You want to create a new field fullName in your aggregation pipeline that combines firstName and lastName with a space, but both names should be in uppercase. Which expression correctly achieves this?

hard

A. {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]}

B. {$toLower: {$concat: ["$firstName", " ", "$lastName"]}}

C. {$concat: ["$firstName", " ", "$lastName"]}

D. {$toLower: {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]}}

Solution

1. Step 1: Understand requirement for uppercase names

   Both firstName and lastName must be converted to uppercase individually before concatenation.

2. Step 2: Analyze each option's logic

   {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]} applies $toUpper to each field separately, then concatenates with space. {$toLower: {$concat: ["$firstName", " ", "$lastName"]}} concatenates first, then lowercases whole string, resulting in lowercase. {$concat: ["$firstName", " ", "$lastName"]} does no uppercase. {$toLower: {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]}} converts final string to lowercase, which is wrong.

3. Final Answer:

   {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]} -> Option A

4. Quick Check:

   Uppercase each name then join with space [OK]

Hint: Uppercase each name before concat for correct result [OK]

Common Mistakes:

• Uppercasing after concatenation changes space too
• Not applying $toUpper to both fields
• Using $toLower mistakenly