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.

Practice

1.

What does CRUD stand for in Cloud Firestore operations?

easy

A. Connect, Refresh, Upload, Delete

B. Copy, Run, Undo, Download

C. Create, Read, Update, Delete

D. Create, Remove, Update, Download

Solution

1. Step 1: Understand CRUD acronym meaning

   CRUD is a common term in databases meaning the four basic operations on data.

2. Step 2: Match each letter to the correct operation

   C = Create, R = Read, U = Update, D = Delete are the standard meanings.

3. Final Answer:

   Create, Read, Update, Delete -> Option C

4. Quick Check:

   CRUD = Create, Read, Update, Delete [OK]

Hint: Remember CRUD as basic data actions: create, read, update, delete [OK]

Common Mistakes:

• Confusing CRUD letters with unrelated words
• Mixing up 'Remove' with 'Delete'
• Thinking CRUD includes 'Download'

2.

Which Flutter code snippet correctly adds a new document to a Firestore collection named users?

FirebaseFirestore.instance.collection('users').____({'name': 'Alice'});

easy

A. update

B. add

C. delete

D. get

Solution

1. Step 1: Identify method to add new document

   The add method creates a new document with given data in the collection.

2. Step 2: Confirm other methods' purposes

   update modifies existing documents, delete removes documents, get reads data.

3. Final Answer:

   add -> Option B

4. Quick Check:

   Adding new document uses add() [OK]

Hint: Use add() to create new documents in a collection [OK]

Common Mistakes:

• Using update() to add new documents
• Confusing get() with add()
• Trying to delete() when adding data

3.

What will be the output of this Flutter Firestore code snippet?

var doc = await FirebaseFirestore.instance.collection('tasks').doc('task1').get();
print(doc.exists);

Assuming the document task1 exists in the tasks collection.

medium

A. true

B. false

C. null

D. Throws an error

Solution

1. Step 1: Understand doc.get() behavior

   The get() method fetches the document snapshot from Firestore.

2. Step 2: Check the exists property

   If the document exists, doc.exists returns true; otherwise false.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Document exists property = true [OK]

Hint: doc.exists is true if document is found in Firestore [OK]

Common Mistakes:

• Assuming doc.exists returns document data
• Expecting false even if document exists
• Not awaiting the get() call

4.

Identify the error in this Flutter Firestore update code:

FirebaseFirestore.instance.collection('users').doc('user1').update({'age': 30});

Assuming the document user1 does not exist.

medium

A. No error, update creates document if missing

B. Update silently fails without error

C. Syntax error in update method call

D. Throws an error because document does not exist

Solution

1. Step 1: Understand update() behavior on missing document

   The update() method requires the document to exist; otherwise, it throws an error.

2. Step 2: Confirm no syntax errors

   The syntax is correct, so error is due to missing document.

3. Final Answer:

   Throws an error because document does not exist -> Option D

4. Quick Check:

   update() fails if document missing [OK]

Hint: update() needs existing document; else error occurs [OK]

Common Mistakes:

• Thinking update() creates new documents
• Confusing update() with set()
• Ignoring async/await for update()

5.

You want to delete a Firestore document only if it exists to avoid errors. Which Flutter code snippet correctly does this?

var docRef = FirebaseFirestore.instance.collection('orders').doc('order1');
// What code to safely delete?

hard

A. docRef.get().then((doc) { if (doc.exists) docRef.delete(); });

B. docRef.delete();

C. docRef.update({'deleted': true});

D. docRef.set(null);

Solution

1. Step 1: Check document existence before deleting

   Use get() to fetch the document snapshot and check exists.

2. Step 2: Call delete() only if document exists

   This avoids errors from deleting non-existing documents.

3. Final Answer:

   docRef.get().then((doc) { if (doc.exists) docRef.delete(); }); -> Option A

4. Quick Check:

   Check exists before delete to avoid errors [OK]

Hint: Always check doc.exists before delete() to prevent errors [OK]

Common Mistakes:

• Calling delete() without checking existence
• Using update() to delete documents
• Setting document to null instead of deleting