$eq for equality in MongoDB - Mini Project: Build & Apply

$eq for equality in MongoDB queries

📖 Scenario: You are managing a small library database. You want to find books by a specific author.

🎯 Goal: Build a MongoDB query using the $eq operator to find all books where the author matches a given name.

📋 What You'll Learn

Create a collection named books with sample book documents

Define a variable authorName with the exact author name to search

Write a MongoDB query using $eq to find books by authorName

Complete the query with a projection to show only the title and author fields

💡 Why This Matters

🌍 Real World

Finding specific records in a database by matching exact values is common in apps like libraries, stores, and user management systems.

💼 Career

Knowing how to use <code>$eq</code> helps you filter data efficiently in MongoDB, a popular NoSQL database used in many companies.

1

Create the books collection with sample data

Create a variable called books that is a list of three documents with these exact entries: { title: "The Hobbit", author: "J.R.R. Tolkien" }, { title: "1984", author: "George Orwell" }, and { title: "Animal Farm", author: "George Orwell" }.

MongoDB

# Create the books collection with three documents
# Your code here

Hint

Use a list of dictionaries to represent the collection.

2

Define the author name to search

Create a variable called authorName and set it to the string "George Orwell".

MongoDB

books = [
  { "title": "The Hobbit", "author": "J.R.R. Tolkien" },
  { "title": "1984", "author": "George Orwell" },
  { "title": "Animal Farm", "author": "George Orwell" }
]
# Define the authorName variable
# Your code here

Hint

Set the variable exactly as shown.

3

Write the MongoDB query using $eq

Create a variable called query and set it to a dictionary that uses $eq to match the author field to the variable authorName. The query should look like { "author": { "$eq": authorName } }.

MongoDB

books = [
  { "title": "The Hobbit", "author": "J.R.R. Tolkien" },
  { "title": "1984", "author": "George Orwell" },
  { "title": "Animal Farm", "author": "George Orwell" }
]
authorName = "George Orwell"
# Create the query using $eq
# Your code here

Hint

Use the $eq operator inside the query dictionary.

4

Add the projection to show only title and author

Create a variable called projection and set it to a dictionary that includes "title": 1 and "author": 1 to show only these fields in the query result.

MongoDB

books = [
  { "title": "The Hobbit", "author": "J.R.R. Tolkien" },
  { "title": "1984", "author": "George Orwell" },
  { "title": "Animal Farm", "author": "George Orwell" }
]
authorName = "George Orwell"
query = { "author": { "$eq": authorName } }
# Add the projection to show only title and author
# Your code here

Hint

Use 1 to include fields in the projection.

Practice

1. What does the $eq operator do in a MongoDB query?

easy

A. It sorts documents in ascending order.

B. It deletes documents from the collection.

C. It updates documents with new values.

D. It matches documents where a field is equal to a specified value.

Solution

1. Step 1: Understand the purpose of $eq

   The $eq operator is used to filter documents where a field exactly matches a given value.

2. Step 2: Compare with other options

   Sorting, updating, and deleting are different operations and not related to $eq.

3. Final Answer:

   It matches documents where a field is equal to a specified value. -> Option D

4. Quick Check:

   $eq means equality match [OK]

Hint: Remember: $eq means 'equals' in queries [OK]

Common Mistakes:

• Confusing $eq with sorting or updating operators
• Thinking $eq modifies data instead of filtering
• Assuming $eq works for inequality

2. Which of the following is the correct syntax to find documents where the field age equals 30 using $eq?

easy

A. { age: { $eq: 30 } }

B. { $eq: { age: 30 } }

C. { age: $eq: 30 }

D. { age == 30 }

Solution

1. Step 1: Recall MongoDB query syntax for $eq

   The correct syntax uses the field name as key and an object with $eq as key and the value to match as value: { age: { $eq: 30 } }.

2. Step 2: Identify incorrect syntax

   Options B, C, and D do not follow MongoDB query syntax rules and will cause errors.

3. Final Answer:

   { age: { $eq: 30 } } -> Option A

4. Quick Check:

   Field: { $eq: value } format [OK]

Hint: Use { field: { $eq: value } } for equality [OK]

Common Mistakes:

• Placing $eq outside the field key
• Using double colons or wrong operators
• Using JavaScript equality syntax instead of MongoDB

3. Given the collection users with documents:

{ "name": "Alice", "age": 25 }, { "name": "Bob", "age": 30 }, { "name": "Carol", "age": 30 }

What will the query db.users.find({ age: { $eq: 30 } }) return?

medium

A. [{ "name": "Bob", "age": 30 }, { "name": "Carol", "age": 30 }]

B. [] (empty array)

C. [{ "name": "Alice", "age": 25 }]

D. Syntax error

Solution

1. Step 1: Understand the query condition

   The query looks for documents where age equals 30.

2. Step 2: Check documents matching the condition

   Bob and Carol have age 30, so both documents match and will be returned.

3. Final Answer:

   [{ "name": "Bob", "age": 30 }, { "name": "Carol", "age": 30 }] -> Option A

4. Quick Check:

   age == 30 returns Bob and Carol [OK]

Hint: Look for documents where field equals value [OK]

Common Mistakes:

• Returning documents with age not equal to 30
• Expecting only one document instead of all matches
• Thinking $eq causes syntax error

4. You wrote the query db.products.find({ price: $eq: 100 }) but it gives an error. What is wrong?

medium

A. Field name price is invalid

B. Missing curly braces around $eq: 100 value

C. You cannot use $eq with numbers

D. The collection name products is incorrect

Solution

1. Step 1: Check the query syntax

   The $eq operator must be inside an object as the value for the field key, like { price: { $eq: 100 } }.

2. Step 2: Identify the missing braces

   The query is missing curly braces around $eq: 100, causing a syntax error.

3. Final Answer:

   Missing curly braces around $eq: 100 value -> Option B

4. Quick Check:

   Use { field: { $eq: value } } syntax [OK]

Hint: Always wrap $eq and value in braces { } [OK]

Common Mistakes:

• Omitting braces around $eq operator
• Assuming $eq works without object syntax
• Blaming collection or field name incorrectly

5. You want to find documents in orders where the status is exactly "shipped" and the quantity is 10. Which query correctly uses $eq for both conditions?

hard

A. db.orders.find({ status: "shipped", quantity: 10 })

B. db.orders.find({ $eq: { status: "shipped", quantity: 10 } })

C. db.orders.find({ status: { $eq: "shipped" }, quantity: { $eq: 10 } })

D. db.orders.find({ status == "shipped", quantity == 10 })

Solution

1. Step 1: Use $eq for each field separately

   Each field must have its own $eq operator inside an object: { status: { $eq: "shipped" } } and { quantity: { $eq: 10 } }.

2. Step 2: Combine conditions in one query object

   Put both conditions inside the find query object to filter documents matching both.

3. Final Answer:

   db.orders.find({ status: { $eq: "shipped" }, quantity: { $eq: 10 } }) -> Option C

4. Quick Check:

   Use separate $eq for each field [OK]

Hint: Use separate $eq for each field in the query [OK]

Common Mistakes:

• Trying to use one $eq for multiple fields
• Using JavaScript equality operators in query
• Omitting $eq and relying on implicit equality