Delete with filter conditions in MongoDB - Mini Project: Build & Apply

Delete Documents with Filter Conditions in MongoDB

📖 Scenario: You manage a small online bookstore database. Over time, some books become outdated or discontinued. You want to clean up the database by deleting books that meet certain conditions.

🎯 Goal: Build a MongoDB query to delete documents from the books collection using filter conditions.

📋 What You'll Learn

Create a books collection with specific book documents

Add a filter condition variable to select books published before 2010

Write a delete query using the filter condition to remove old books

Confirm the delete query targets the correct documents

💡 Why This Matters

🌍 Real World

Cleaning up outdated or irrelevant data in a database to keep it accurate and efficient.

💼 Career

Database administrators and developers often need to delete records based on conditions to maintain data quality.

1

Create the books collection with sample documents

Create a variable called books and assign it an array with these exact documents: { title: 'Learn MongoDB', year: 2015 }, { title: 'Old Book', year: 2005 }, { title: 'Modern Database', year: 2020 }

MongoDB

const books = [
  // Your code here
];

Hint

Use an array of objects with the exact titles and years given.

2

Create a filter condition for books published before 2010

Create a variable called filterCondition and set it to { year: { $lt: 2010 } } to select books published before 2010

MongoDB

const books = [
  { title: 'Learn MongoDB', year: 2015 },
  { title: 'Old Book', year: 2005 },
  { title: 'Modern Database', year: 2020 }
];

// Your code here

Hint

Use MongoDB's $lt operator to filter years less than 2010.

3

Write the delete query using the filter condition

Write a line that calls db.books.deleteMany(filterCondition) to delete all books matching the filter condition

MongoDB

const books = [
  { title: 'Learn MongoDB', year: 2015 },
  { title: 'Old Book', year: 2005 },
  { title: 'Modern Database', year: 2020 }
];

const filterCondition = { year: { $lt: 2010 } };

// Your code here

Hint

Use deleteMany with the filter condition variable.

4

Confirm the delete query targets the correct documents

Add a line that calls db.books.find() to check remaining documents after deletion

MongoDB

const books = [
  { title: 'Learn MongoDB', year: 2015 },
  { title: 'Old Book', year: 2005 },
  { title: 'Modern Database', year: 2020 }
];

const filterCondition = { year: { $lt: 2010 } };

db.books.deleteMany(filterCondition);

// Your code here

Hint

Use find() to see which documents remain after deletion.

Practice

1. What does the deleteMany method do in MongoDB when used with a filter condition?

easy

A. Deletes the entire collection regardless of the filter.

B. Deletes only the first document in the collection.

C. Deletes all documents that match the filter condition.

D. Updates documents instead of deleting them.

Solution

1. Step 1: Understand deleteMany purpose

   deleteMany is designed to remove multiple documents matching a filter.

2. Step 2: Apply filter condition effect

   Only documents matching the filter are deleted, not the entire collection.

3. Final Answer:

   Deletes all documents that match the filter condition. -> Option C

4. Quick Check:

   deleteMany removes all matching docs [OK]

Hint: Remember: deleteMany removes all matching documents [OK]

Common Mistakes:

• Confusing deleteMany with deleteOne
• Thinking deleteMany deletes entire collection
• Assuming deleteMany updates documents

2. Which of the following is the correct syntax to delete one document where the field status equals "inactive" in MongoDB?

easy

A. db.collection.deleteOne({status: "inactive"})

B. db.collection.delete({status: "inactive"})

C. db.collection.removeOne({status: "inactive"})

D. db.collection.deleteMany({status == "inactive"})

Solution

1. Step 1: Identify correct method for deleting one document

   The method to delete a single document is deleteOne.

2. Step 2: Check filter syntax correctness

   The filter uses a key-value pair with colon, not double equals or other syntax.

3. Final Answer:

   db.collection.deleteOne({status: "inactive"}) -> Option A

4. Quick Check:

   Correct method and filter syntax = db.collection.deleteOne({status: "inactive"}) [OK]

Hint: Use deleteOne with colon syntax for filters [OK]

Common Mistakes:

• Using delete instead of deleteOne
• Using == instead of : in filter
• Using removeOne which does not exist

3. Given the collection users with documents:
{name: "Alice", age: 30}
{name: "Bob", age: 25}
{name: "Charlie", age: 30}
What will be the result after running db.users.deleteMany({age: 30})?

medium

A. Only Alice's document is deleted.

B. Both Alice's and Charlie's documents are deleted.

C. Only Bob's document is deleted.

D. No documents are deleted.

Solution

1. Step 1: Identify filter condition effect

   The filter {age: 30} matches documents where age is exactly 30.

2. Step 2: Determine matching documents

   Alice and Charlie both have age 30, so both match and will be deleted by deleteMany.

3. Final Answer:

   Both Alice's and Charlie's documents are deleted. -> Option B

4. Quick Check:

   deleteMany removes all matching docs = Both Alice's and Charlie's documents are deleted. [OK]

Hint: deleteMany removes all docs matching filter, not just one [OK]

Common Mistakes:

• Deleting only one document with deleteMany
• Deleting documents not matching filter
• Confusing age 25 with 30

4. You run the command db.orders.deleteOne({orderId: 12345}) but no documents are deleted. What could be the problem?

medium

A. MongoDB does not support deleteOne method.

B. deleteOne deletes all documents, so it should have deleted more.

C. You must use deleteMany to delete any documents.

D. The filter field name or value might be incorrect or missing in documents.

Solution

1. Step 1: Understand deleteOne behavior

   deleteOne deletes only one document matching the filter if it exists.

2. Step 2: Check filter correctness

   If no document matches {orderId: 12345}, nothing is deleted. The field or value may be wrong or missing.

3. Final Answer:

   The filter field name or value might be incorrect or missing in documents. -> Option D

4. Quick Check:

   No matching document means no deletion [OK]

Hint: Check filter matches existing documents before deleting [OK]

Common Mistakes:

• Assuming deleteOne deletes all documents
• Using deleteMany when deleteOne is intended
• Thinking deleteOne does not exist

5. You want to delete all documents from the products collection where the stock field is less than or equal to 0. Which MongoDB command correctly achieves this?

hard

A. db.products.deleteMany({stock: {$lte: 0}})

B. db.products.deleteOne({stock <= 0})

C. db.products.remove({stock: {$lt: 0}})

D. db.products.deleteMany({stock: {$gte: 0}})

Solution

1. Step 1: Identify correct operator for less than or equal

   The MongoDB operator for less than or equal is $lte.

2. Step 2: Choose correct method and filter syntax

   deleteMany deletes all matching documents; filter must use {stock: {$lte: 0}}.

3. Final Answer:

   db.products.deleteMany({stock: {$lte: 0}}) -> Option A

4. Quick Check:

   Use $lte with deleteMany for all matching docs [OK]

Hint: Use $lte operator inside deleteMany filter for <= condition [OK]

Common Mistakes:

• Using incorrect comparison syntax like stock <= 0
• Using deleteOne instead of deleteMany for multiple docs
• Using $gte instead of $lte