Delete all documents in collection in MongoDB - Mini Project: Build & Apply

Delete All Documents in a MongoDB Collection

📖 Scenario: You are managing a MongoDB database for a small online bookstore. Sometimes, you need to clear out all the book records to reset the collection before adding new data.

🎯 Goal: Learn how to delete all documents from a MongoDB collection using a simple command.

📋 What You'll Learn

Create a MongoDB collection named books with some sample documents.

Set up a variable to reference the books collection.

Use the correct MongoDB command to delete all documents in the books collection.

Confirm the deletion command is correctly structured.

💡 Why This Matters

🌍 Real World

Clearing out old or test data from a MongoDB collection is common when resetting databases or cleaning up after tests.

💼 Career

Database administrators and backend developers often need to delete data efficiently and safely using commands like 'deleteMany'.

1

Create the books collection with sample documents

Create a MongoDB collection called books and insert these exact documents: { title: "Book A", author: "Author 1" }, { title: "Book B", author: "Author 2" }, and { title: "Book C", author: "Author 3" }.

MongoDB

// Insert the sample documents into the books collection
// Your code here

Hint

Use db.books.insertMany([...]) to add multiple documents at once.

2

Set a variable to reference the books collection

Create a variable called collection and set it to db.books to reference the books collection.

MongoDB

db.books.insertMany([
  { title: "Book A", author: "Author 1" },
  { title: "Book B", author: "Author 2" },
  { title: "Book C", author: "Author 3" }
])

// Create a variable named collection to reference db.books
// Your code here

Hint

Use const collection = db.books to create the variable.

3

Delete all documents from the books collection

Use the deleteMany method on the collection variable with an empty filter {} to delete all documents in the collection.

MongoDB

db.books.insertMany([
  { title: "Book A", author: "Author 1" },
  { title: "Book B", author: "Author 2" },
  { title: "Book C", author: "Author 3" }
])

const collection = db.books

// Use collection.deleteMany({}) to delete all documents
// Your code here

Hint

Use collection.deleteMany({}) to remove all documents.

4

Confirm the deletion command is complete

Add a semicolon ; at the end of the collection.deleteMany({}) command to complete the statement.

MongoDB

db.books.insertMany([
  { title: "Book A", author: "Author 1" },
  { title: "Book B", author: "Author 2" },
  { title: "Book C", author: "Author 3" }
])

const collection = db.books

collection.deleteMany({})
// Add a semicolon at the end of the deleteMany command
// Your code here

Hint

Remember to end statements with a semicolon ; in JavaScript.

Practice

1. What does the MongoDB command db.collection.deleteMany({}) do?

easy

A. Deletes all documents in the collection but keeps the collection itself.

B. Deletes the entire collection including its structure.

C. Deletes only the first document in the collection.

D. Updates all documents in the collection to empty values.

Solution

1. Step 1: Understand the deleteMany method

   The deleteMany method removes documents matching the filter. An empty filter {} matches all documents.

2. Step 2: Effect of using an empty filter

   Using {} deletes all documents but does not remove the collection itself.

3. Final Answer:

   Deletes all documents in the collection but keeps the collection itself. -> Option A

4. Quick Check:

   deleteMany({}) removes all documents [OK]

Hint: Empty filter {} deletes all documents, collection stays [OK]

Common Mistakes:

• Thinking deleteMany({}) drops the collection
• Confusing deleteMany with deleteOne
• Assuming documents are updated, not deleted

2. Which of the following is the correct syntax to delete all documents from a MongoDB collection named users?

easy

A. db.users.deleteAll({})

B. db.users.deleteMany()

C. db.users.deleteMany({})

D. db.users.removeAll()

Solution

1. Step 1: Identify the correct method and syntax

   The method to delete multiple documents is deleteMany, which requires a filter argument.

2. Step 2: Use an empty filter to delete all documents

   Passing an empty object {} as the filter deletes all documents in the collection.

3. Final Answer:

   db.users.deleteMany({}) -> Option C

4. Quick Check:

   Correct syntax includes empty filter {} [OK]

Hint: deleteMany needs a filter; use {} for all documents [OK]

Common Mistakes:

• Omitting the filter argument in deleteMany
• Using non-existent methods like deleteAll or removeAll
• Using deleteMany without parentheses

3. Given the collection products with 5 documents, what will be the result of running db.products.deleteMany({}) followed by db.products.find().toArray()?

medium

A. Returns an empty array [] because all documents are deleted.

B. Returns all 5 documents because deleteMany does not delete anything.

C. Returns an error because deleteMany requires a filter.

D. Returns only the first document because deleteMany deletes one document.

Solution

1. Step 1: Effect of deleteMany({}) on documents

   Using deleteMany({}) deletes all documents in the collection.

2. Step 2: Result of find() after deletion

   After deletion, find() returns no documents, so toArray() returns an empty array.

3. Final Answer:

   Returns an empty array [] because all documents are deleted. -> Option A

4. Quick Check:

   deleteMany({}) empties collection, find() returns [] [OK]

Hint: deleteMany({}) clears all documents; find() shows empty [OK]

Common Mistakes:

• Expecting documents to remain after deleteMany({})
• Thinking deleteMany deletes only one document
• Assuming deleteMany without filter causes error

4. You run db.orders.deleteMany() to delete all documents in the orders collection but get an error. What is the likely cause?

medium

A. The collection orders does not exist.

B. You need to use removeMany instead.

C. deleteMany cannot delete multiple documents.

D. Missing filter argument in deleteMany method.

Solution

1. Step 1: Check deleteMany method requirements

   The deleteMany method requires a filter argument; omitting it causes a syntax error.

2. Step 2: Confirm correct usage to delete all documents

   To delete all documents, pass an empty filter {} as argument.

3. Final Answer:

   Missing filter argument in deleteMany method. -> Option D

4. Quick Check:

   deleteMany needs a filter argument [OK]

Hint: Always provide filter to deleteMany, use {} for all [OK]

Common Mistakes:

• Calling deleteMany without parentheses or filter
• Confusing deleteMany with removeMany (not a method)
• Assuming collection absence causes this error

5. You want to clear all documents from the logs collection but keep the collection and its indexes intact. Which command should you use?

hard

A. db.logs.deleteOne({})

B. db.logs.deleteMany({})

C. db.logs.removeAll()

D. db.logs.drop()

Solution

1. Step 1: Understand difference between drop and deleteMany

   drop() removes the entire collection and indexes, while deleteMany({}) removes all documents but keeps collection and indexes.

2. Step 2: Choose command that clears documents but preserves collection

   Using deleteMany({}) clears all documents without dropping the collection or indexes.

3. Final Answer:

   db.logs.deleteMany({}) -> Option B

4. Quick Check:

   deleteMany({}) clears docs, keeps collection [OK]

Hint: Use deleteMany({}) to clear data, drop() removes collection [OK]

Common Mistakes:

• Using drop() which deletes collection and indexes
• Using non-existent removeAll() method
• Using deleteOne() which deletes only one document