Deleting documents in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does deleting a document in Firebase Firestore do?

Deleting a document removes it completely from the database, so it no longer exists or can be read.

beginner

Which Firebase method is used to delete a document?

The delete() method on a document reference is used to delete a document.

intermediate

What happens if you try to delete a document that does not exist?

Firebase does not throw an error; the delete operation is simply ignored because the document is already absent.

intermediate

Can deleting a document in Firestore affect its subcollections?

No, deleting a document does not delete its subcollections. Subcollections remain unless deleted separately.

intermediate

Why is it important to handle errors when deleting documents in Firebase?

Handling errors ensures your app can respond to issues like permission denials or network problems gracefully.

Which method deletes a document in Firebase Firestore?

A delete()

B remove()

C destroy()

D erase()

The correct method to delete a document is delete().

What happens to subcollections when you delete a Firestore document?

A They are deleted automatically

B They remain intact

C They are archived

D They are moved to a backup

Deleting a document does not delete its subcollections; they remain unless deleted separately.

If you delete a non-existent document in Firestore, what happens?

A An error is thrown

B A new document is created

C The operation is ignored

D The database crashes

Deleting a non-existent document does not cause an error; the operation is simply ignored.

Why should you handle errors when deleting documents in Firebase?

A To handle permission or network issues

B To improve app responsiveness

C To prevent data loss

D To speed up deletion

Handling errors helps your app respond properly to permission denials or network problems.

Which of these is NOT a valid reason to delete a document?

A Correcting mistakes

B Freeing up storage

C Removing outdated data

D Backing up data

Backing up data is not a reason to delete a document; it involves copying data, not removing it.

Explain the process and effects of deleting a document in Firebase Firestore.

Think about what happens to the document and its related data.

Describe why error handling is important when deleting documents in Firebase.

Consider what can go wrong during deletion.

Practice

1. What does the delete() method do in Firebase Firestore?

easy

A. Reads data from a document

B. Adds a new document to the database

C. Updates fields in an existing document

D. Removes a specific document from the database

Solution

1. Step 1: Understand the purpose of delete()

   The delete() method is used to remove a document from Firestore.

2. Step 2: Compare with other operations

   Adding, updating, and reading documents use different methods like set(), update(), and get().

3. Final Answer:

   Removes a specific document from the database -> Option D

4. Quick Check:

   delete() removes documents [OK]

Hint: Delete removes documents, not adds or updates [OK]

Common Mistakes:

• Confusing delete() with set() or update()
• Thinking delete() reads data
• Assuming delete() removes collections

2. Which of the following is the correct syntax to delete a document with ID 'doc1' from a collection 'users' in Firestore?

easy

A. db.collection('users').doc('doc1').remove()

B. db.collection('users').delete('doc1')

C. db.collection('users').doc('doc1').delete()

D. db.delete('users/doc1')

Solution

1. Step 1: Identify correct method to delete a document

   The method to delete a document is delete() called on a document reference.

2. Step 2: Check syntax for document reference

   Use collection('users').doc('doc1') to get the document, then call delete().

3. Final Answer:

   db.collection('users').doc('doc1').delete() -> Option C

4. Quick Check:

   Use doc().delete() to remove documents [OK]

Hint: Use doc('id').delete() to remove a document [OK]

Common Mistakes:

• Using remove() instead of delete()
• Calling delete() on collection instead of doc
• Wrong method chaining order

3. What will the following code output if the document 'doc1' exists and is deleted successfully?

db.collection('users').doc('doc1').delete()
  .then(() => console.log('Deleted'))
  .catch(() => console.log('Error'))

medium

A. Error

B. Deleted

C. Nothing

D. SyntaxError

Solution

1. Step 1: Understand promise handling in delete()

   The delete() method returns a promise that resolves on success and rejects on failure.

2. Step 2: Analyze the then/catch handlers

   If deletion succeeds, then() runs and logs 'Deleted'. If it fails, catch() logs 'Error'.

3. Final Answer:

   Deleted -> Option B

4. Quick Check:

   Successful delete logs 'Deleted' [OK]

Hint: Successful delete triggers then(), logs 'Deleted' [OK]

Common Mistakes:

• Assuming delete() is synchronous
• Ignoring promise handling
• Expecting output without then()

4. Identify the error in this code snippet that tries to delete a document:

const docRef = db.collection('users').doc('doc1');
docRef.delete;
console.log('Deleted');

medium

A. Missing parentheses after delete method call

B. Wrong collection name

C. docRef is not defined

D. delete is not a method

Solution

1. Step 1: Check method invocation syntax

   The delete method must be called with parentheses: delete().

2. Step 2: Identify impact of missing parentheses

   Without parentheses, the method is not executed, so the document is not deleted.

3. Final Answer:

   Missing parentheses after delete method call -> Option A

4. Quick Check:

   Call delete() with () to execute [OK]

Hint: Always use parentheses to call delete() method [OK]

Common Mistakes:

• Forgetting () after delete
• Assuming delete is a property, not a method
• Ignoring promise handling

5. You want to delete multiple documents with IDs ['doc1', 'doc2', 'doc3'] from the 'users' collection. Which approach correctly deletes all documents and logs 'All deleted' after completion?

hard

A. Use Promise.all with delete promises:Promise.all(['doc1','doc2','doc3'].map(id => db.collection('users').doc(id).delete()))
.then(() => console.log('All deleted'))

B. Call delete() on the collection:db.collection('users').delete();
console.log('All deleted');

C. Use a loop with await:for (const id of ['doc1','doc2','doc3']) {
await db.collection('users').doc(id).delete();
}
console.log('All deleted');

D. Delete documents without waiting:['doc1','doc2','doc3'].forEach(id => db.collection('users').doc(id).delete());
console.log('All deleted');

Solution

1. Step 1: Understand asynchronous deletion of multiple documents

   Deleting multiple documents returns promises; to know when all finish, use Promise.all().

2. Step 2: Evaluate each option's correctness

   Use Promise.all with delete promises:
   

   Promise.all(['doc1','doc2','doc3'].map(id => db.collection('users').doc(id).delete()))
     .then(() => console.log('All deleted'))

   uses Promise.all() with delete promises to wait for all deletions concurrently before logging. B calls invalid delete() on collection. C uses await loop without async function, causing syntax error. D logs without waiting for completion.

3. Final Answer:

   Use Promise.all with delete promises -> Option A

4. Quick Check:

   Promise.all waits for all deletes before logging [OK]

Hint: Use Promise.all to wait for all deletes before logging [OK]

Common Mistakes:

• Trying to delete entire collection directly
• Not waiting for all deletes before logging
• Using forEach without async/await or Promise.all