Cloud Firestore CRUD in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Firestore Document Creation Behavior

What will be the result on Firestore after running this Flutter code snippet to add a new document?

Flutter

FirebaseFirestore.instance.collection('users').add({'name': 'Alice', 'age': 30});

A The document with ID 'users' is overwritten with the new data.

B A new document with an auto-generated ID is created in the 'users' collection with fields 'name' and 'age'.

C An error occurs because the document ID is missing.

D No document is created because 'add' only updates existing documents.

💡 Hint

Remember that 'add' creates a new document with a random ID.

The 'add' method creates a new document with a unique ID inside the specified collection. It does not overwrite or require an ID.

📝 Syntax

intermediate

Correct Firestore Document Update Syntax

Which option correctly updates the 'age' field of a Firestore document with ID 'user123' in the 'users' collection?

Flutter

FirebaseFirestore.instance.collection('users').doc('user123').???({'age': 31});

A update

B set

C add

D create

💡 Hint

Use the method that modifies existing fields without overwriting the whole document.

'update' changes only specified fields in an existing document. 'set' replaces the whole document unless merge is true.

❓ lifecycle

advanced

Firestore Stream Listener Behavior

What happens in the Flutter app when using this Firestore stream to listen to 'users' collection changes?

Flutter

FirebaseFirestore.instance.collection('users').snapshots().listen((snapshot) {
  print('Documents count: ${snapshot.docs.length}');
});

A The listener prints the document count only once when the app starts.

B The listener prints the document count only when a new document is added, not when updated or deleted.

C The listener causes an error because snapshots() returns a Future, not a Stream.

D The listener prints the current number of documents every time the 'users' collection changes in Firestore.

💡 Hint

Streams update on any change in the collection including add, update, or delete.

The snapshots() method returns a stream that emits a new snapshot whenever the collection changes in any way.

🔧 Debug

advanced

Fix Firestore Document Deletion Error

This Flutter code tries to delete a Firestore document but will not work as expected. What is the cause?

Flutter

FirebaseFirestore.instance.collection('users').doc().delete();

A The doc() method is called without an ID, so it targets a non-existing document.

B The delete() method requires a callback function to confirm deletion.

C The collection name 'users' is invalid and causes the error.

D The code is correct and should not cause any error.

💡 Hint

Deleting requires specifying the exact document ID.

Calling doc() without an ID creates a reference to a document with a random ID that does not exist, so delete() targets the wrong document.

🧠 Conceptual

expert

Firestore Offline Persistence Behavior

Which statement best describes Firestore offline persistence in a Flutter app?

A Firestore does not support offline data; all reads and writes require internet connection.

B Firestore requires manual syncing of data after reconnecting to the internet.

C Firestore caches data locally and syncs changes automatically when the device reconnects to the internet.

D Firestore deletes local data cache every time the app restarts.

💡 Hint

Think about how apps work when you lose internet but still see data.

Firestore enables offline persistence by caching data locally and syncing changes automatically once online.