deleteMany method in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the deleteMany() method do in MongoDB?

It removes all documents from a collection that match a given filter condition.

beginner

How do you specify which documents to delete using deleteMany()?

You provide a filter object that describes the criteria documents must meet to be deleted.

intermediate

What does deleteMany({}) do?

It deletes all documents in the collection because the empty filter matches everything.

intermediate

What is the return value of deleteMany()?

It returns a result object containing the number of documents deleted, accessible via deletedCount.

beginner

Why should you be careful when using deleteMany() without a filter?

Because it will delete all documents in the collection, which might cause data loss if unintended.

What does deleteMany({ age: { $lt: 18 } }) do?

A Deletes all documents where age is less than 18

B Deletes all documents where age is greater than 18

C Deletes all documents regardless of age

D Deletes no documents

The filter { age: { $lt: 18 } } matches documents with age less than 18, so those are deleted.

What will happen if you call deleteMany({}) on a collection?

A Deletes no documents

B Deletes documents with null values

C Deletes all documents

D Throws an error

An empty filter matches all documents, so all documents are deleted.

Which property tells you how many documents were deleted after deleteMany()?

A deletedCount

B countDeleted

C deletedDocuments

D deleteCount

The result object from deleteMany() includes a property called deletedCount showing how many documents were removed.

If you want to delete documents where the field 'status' equals 'inactive', what filter should you use?

A {}

B { status: { $eq: 'active' } }

C { inactive: true }

D { status: 'inactive' }

The filter { status: 'inactive' } matches documents where status is exactly 'inactive'.

Is it possible to delete documents using deleteMany() without specifying a filter?

A No, filter is required

B Yes, it deletes all documents

C Yes, but it deletes none

D No, it throws an error

Calling deleteMany() with an empty filter deletes all documents in the collection.

Explain how the deleteMany() method works in MongoDB and what happens if you use an empty filter.

Think about how you tell MongoDB which documents to remove.

Describe the importance of the deletedCount property after calling deleteMany().

Consider how you know if your delete worked.

Practice

1. What does the deleteMany method do in MongoDB?

easy

A. Inserts multiple documents into a collection.

B. Deletes only one document regardless of the filter.

C. Removes all documents that match a given filter.

D. Updates multiple documents based on a filter.

Solution

1. Step 1: Understand the purpose of deleteMany

   The deleteMany method is designed to remove multiple documents that match a specified filter in a collection.

2. Step 2: Compare with other methods

   Unlike deleteOne which deletes a single document, deleteMany deletes all matching documents. It does not update or insert documents.

3. Final Answer:

   Removes all documents that match a given filter. -> Option C

4. Quick Check:

   deleteMany removes multiple matching documents = D [OK]

Hint: deleteMany deletes all matching documents, not just one [OK]

Common Mistakes:

• Confusing deleteMany with deleteOne
• Thinking deleteMany updates documents
• Assuming deleteMany inserts documents

2. Which of the following is the correct syntax to delete all documents where status is "inactive" using deleteMany?

easy

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

B. db.collection.deleteMany("status = 'inactive'")

C. db.collection.deleteMany(status == "inactive")

D. db.collection.deleteMany([status: "inactive"])

Solution

1. Step 1: Identify correct filter syntax

   The filter in deleteMany must be a JSON object with key-value pairs, like {status: "inactive"}.

2. Step 2: Check each option

   db.collection.deleteMany({status: "inactive"}) uses correct JSON object syntax. Options B, C, and D use invalid syntax for MongoDB filters.

3. Final Answer:

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

4. Quick Check:

   Filter must be JSON object = A [OK]

Hint: Use JSON object for filter in deleteMany [OK]

Common Mistakes:

• Using string instead of object for filter
• Using comparison operators inside filter incorrectly
• Using array syntax for filter

3. Given the collection users with documents:

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

What will be the result of db.users.deleteMany({age: 25})?

medium

A. Deletes 2 documents where age is 25

B. Deletes 1 document where age is 25

C. Deletes all documents regardless of age

D. No documents deleted

Solution

1. Step 1: Identify matching documents

   Documents with age: 25 are Alice and Carol, so 2 documents match the filter.

2. Step 2: Understand deleteMany behavior

   deleteMany removes all documents matching the filter, so both Alice and Carol will be deleted.

3. Final Answer:

   Deletes 2 documents where age is 25 -> Option A

4. Quick Check:

   deleteMany removes all matching documents = A [OK]

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

Common Mistakes:

• Thinking only one document is deleted
• Assuming deleteMany deletes all documents
• Confusing filter criteria

4. What is wrong with this code snippet?

db.products.deleteMany("{category: 'electronics'}")

medium

A. deleteMany cannot delete documents by category.

B. The collection name is incorrect.

C. Missing semicolon at the end.

D. Filter should be an object, not a string.

Solution

1. Step 1: Check filter argument type

   The filter argument must be a JSON object, but here it is passed as a string.

2. Step 2: Validate other parts

   deleteMany can delete by any filter, semicolon is optional in JS, and collection name is valid.

3. Final Answer:

   Filter should be an object, not a string. -> Option D

4. Quick Check:

   Filter must be object, not string = B [OK]

Hint: Pass filter as object, not string, in deleteMany [OK]

Common Mistakes:

• Passing filter as string instead of object
• Assuming semicolon is mandatory
• Misnaming collection

5. You want to delete all documents from the orders collection where the status is either "cancelled" or "returned". Which deleteMany filter correctly achieves this?

hard

A. db.orders.deleteMany({status: "cancelled" || "returned"})

B. db.orders.deleteMany({status: {$in: ["cancelled", "returned"]}})

C. db.orders.deleteMany({status: {$or: ["cancelled", "returned"]}})

D. db.orders.deleteMany({status: ["cancelled", "returned"]})

Solution

1. Step 1: Understand filter for multiple values

   To match documents where a field equals any value in a list, MongoDB uses the $in operator with an array of values.

2. Step 2: Evaluate each option

   db.orders.deleteMany({status: {$in: ["cancelled", "returned"]}}) correctly uses $in. db.orders.deleteMany({status: "cancelled" || "returned"}) uses JavaScript OR incorrectly inside an object. db.orders.deleteMany({status: {$or: ["cancelled", "returned"]}}) misuses $or inside a field. db.orders.deleteMany({status: ["cancelled", "returned"]}) uses an array directly, which is invalid.

3. Final Answer:

   db.orders.deleteMany({status: {$in: ["cancelled", "returned"]}}) -> Option B

4. Quick Check:

   Use $in for multiple values in filter = C [OK]

Hint: Use $in operator to match multiple values in deleteMany filter [OK]

Common Mistakes:

• Using JavaScript OR inside filter object
• Misusing $or inside a field filter
• Passing array directly as field value