Query filter syntax in MongoDB - Mini Project: Build & Apply

Query Filter Syntax in MongoDB

📖 Scenario: You are managing a small online bookstore database. You want to find books based on certain criteria like author, price, and availability.

🎯 Goal: Build MongoDB queries step-by-step to filter books by author, price, and stock status using query filter syntax.

📋 What You'll Learn

Create a collection named books with sample book documents

Add a filter variable to specify query conditions

Write a MongoDB query using the filter to find matching books

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

💡 Why This Matters

🌍 Real World

Filtering data is a common task in databases to find specific records quickly, such as finding books by a certain author or price range.

💼 Career

Understanding query filter syntax is essential for database administrators, backend developers, and data analysts who work with MongoDB or similar NoSQL databases.

1

Create the books collection with sample documents

Create a variable called books and assign it a list of three book documents with these exact fields and values: { title: "The Great Gatsby", author: "F. Scott Fitzgerald", price: 10, inStock: true }, { title: "1984", author: "George Orwell", price: 15, inStock: false }, and { title: "To Kill a Mockingbird", author: "Harper Lee", price: 12, inStock: true }.

MongoDB

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

Hint

Use a list of dictionaries to represent the books collection with exact field names and values.

2

Create a filter to find books by author "George Orwell"

Create a variable called filter and assign it a dictionary with the key author and value "George Orwell" to filter books by this author.

MongoDB

books = [
    { "title": "The Great Gatsby", "author": "F. Scott Fitzgerald", "price": 10, "inStock": true },
    { "title": "1984", "author": "George Orwell", "price": 15, "inStock": false },
    { "title": "To Kill a Mockingbird", "author": "Harper Lee", "price": 12, "inStock": true }
]
# Create a filter to find books by author "George Orwell"
# Your code here

Hint

Use a dictionary with the key author and value "George Orwell" to create the filter.

3

Write a MongoDB query using the filter to find matching books

Create a variable called query_result and assign it the result of filtering the books list using the filter variable. Use a list comprehension that checks if each book's author matches the filter's author.

MongoDB

books = [
    { "title": "The Great Gatsby", "author": "F. Scott Fitzgerald", "price": 10, "inStock": true },
    { "title": "1984", "author": "George Orwell", "price": 15, "inStock": false },
    { "title": "To Kill a Mockingbird", "author": "Harper Lee", "price": 12, "inStock": true }
]
filter = { "author": "George Orwell" }
# Use a list comprehension to filter books by author
# Your code here

Hint

Use a list comprehension to select books where the author matches the filter's author.

4

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

Create a variable called final_result and assign it a list of dictionaries containing only the title and author fields from each book in query_result. Use a list comprehension to extract these fields.

MongoDB

books = [
    { "title": "The Great Gatsby", "author": "F. Scott Fitzgerald", "price": 10, "inStock": true },
    { "title": "1984", "author": "George Orwell", "price": 15, "inStock": false },
    { "title": "To Kill a Mockingbird", "author": "Harper Lee", "price": 12, "inStock": true }
]
filter = { "author": "George Orwell" }
query_result = [book for book in books if book["author"] == filter["author"]]
# Create a projection to show only title and author
# Your code here

Hint

Use a list comprehension to create a new list with dictionaries containing only title and author from each book.

Practice

1. What does a MongoDB query filter do?
{ age: 25 }

easy

A. Deletes documents where age is 25

B. Updates documents to set age to 25

C. Selects documents where the age field is exactly 25

D. Creates a new document with age 25

Solution

1. Step 1: Understand the role of a query filter

   A query filter in MongoDB is used to find documents that match certain conditions, not to modify or delete them.

2. Step 2: Analyze the filter { age: 25 }

   This filter matches documents where the field age has the value 25 exactly.

3. Final Answer:

   Selects documents where the age field is exactly 25 -> Option C

4. Quick Check:

   Query filter = select matching documents [OK]

Hint: Filters pick matching data, not modify or delete [OK]

Common Mistakes:

• Confusing filter with update or delete commands
• Thinking filter creates or changes documents
• Assuming filter matches partial or range values without operators

2. Which of these is the correct syntax to find documents where score is greater than 80?

easy

A. { score > 80 }

B. { score: gt 80 }

C. { $score: { gt: 80 } }

D. { score: { $gt: 80 } }

Solution

1. Step 1: Recall MongoDB operator syntax

   MongoDB uses JSON objects with operators starting with a dollar sign, like $gt for 'greater than'.

2. Step 2: Check each option

   { score: { $gt: 80 } } uses { score: { $gt: 80 } } which is correct syntax. Others miss the dollar sign or use invalid syntax.

3. Final Answer:

   { score: { $gt: 80 } } -> Option D

4. Quick Check:

   Use $gt for greater than in filters [OK]

Hint: Operators start with $ inside field object [OK]

Common Mistakes:

• Omitting $ before operator
• Using comparison operators like > directly
• Wrong operator names without $

3. Given the collection documents:

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

What will this query return?
db.collection.find({ age: 30 })

medium

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

B. [{ name: "Bob", age: 25 }]

C. [] (empty array)

D. Syntax error

Solution

1. Step 1: Understand the filter condition

   The filter { age: 30 } matches documents where the age field equals 30.

2. Step 2: Identify matching documents

   Documents for Alice and Carol have age 30, so both match. Bob has age 25, so does not match.

3. Final Answer:

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

4. Quick Check:

   Filter matches exact age 30 documents [OK]

Hint: Filter matches exact field values without operators [OK]

Common Mistakes:

• Expecting only one document returned
• Confusing filter with update or delete
• Thinking filter matches partial or range without operators

4. What is wrong with this MongoDB query filter?
db.collection.find({ age: $gt: 20 })

medium

A. Using $gt instead of $gte

B. Missing curly braces around $gt operator

C. Field name should be $age, not age

D. No error, query is correct

Solution

1. Step 1: Check operator syntax in filter

   MongoDB requires operators like $gt to be inside an object as a value for the field key.

2. Step 2: Identify the syntax error

   The query uses { age: $gt: 20 } which is invalid. It should be { age: { $gt: 20 } } with curly braces around $gt: 20.

3. Final Answer:

   Missing curly braces around $gt operator -> Option B

4. Quick Check:

   Operators must be inside braces as field values [OK]

Hint: Put operators inside braces after field name [OK]

Common Mistakes:

• Omitting braces around operator
• Using wrong operator names
• Misplacing $ sign

5. You want to find documents where status is either "active" or "pending". Which filter correctly does this?

hard

A. { status: { $in: ["active", "pending"] } }

B. { status: { $or: ["active", "pending"] } }

C. { $or: { status: "active", status: "pending" } }

D. { status: "active" || "pending" }

Solution

1. Step 1: Understand how to match multiple values

   MongoDB uses $in operator to match a field against any value in a list.

2. Step 2: Analyze each option

   { status: { $in: ["active", "pending"] } } uses { status: { $in: ["active", "pending"] } } which is correct. { status: { $or: ["active", "pending"] } } uses invalid operator $or inside field. { $or: { status: "active", status: "pending" } } has wrong syntax for $or. { status: "active" || "pending" } uses invalid JavaScript syntax.

3. Final Answer:

   { status: { $in: ["active", "pending"] } } -> Option A

4. Quick Check:

   Use $in for matching any value in list [OK]

Hint: Use $in with array for multiple value matches [OK]

Common Mistakes:

• Using $or inside field instead of top-level
• Wrong syntax for $or operator
• Using JavaScript operators inside filter