Document-collection data model in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve all documents in a collection

Given a Firestore collection named users with documents containing fields name and age, what will be the output of this query?

Firebase

const snapshot = await db.collection('users').get();
const result = snapshot.docs.map(doc => doc.data());
console.log(result);

A [{name: 'Alice'}, {name: 'Bob'}]

B [{id: 'user1', name: 'Alice', age: 30}, {id: 'user2', name: 'Bob', age: 25}]

C [{name: 'Alice', age: 30}, {name: 'Bob', age: 25}]

D []

💡 Hint

The doc.data() method returns only the fields inside the document, not the document ID.

The query fetches all documents in the 'users' collection. Mapping with doc.data() returns an array of objects with only the fields, excluding document IDs.

🧠 Conceptual

intermediate

Understanding subcollections in Firestore

Which statement correctly describes subcollections in Firestore?

A Subcollections are only allowed at the root level of the database.

B Subcollections are nested inside documents and can have their own documents and collections.

C Subcollections are stored as fields inside a document as arrays.

D Subcollections automatically merge with parent collections when queried.

💡 Hint

Think about how Firestore organizes data hierarchically.

Subcollections are separate collections nested under a document. They can contain their own documents and further subcollections, allowing hierarchical data organization.

📝 Syntax

advanced

Correct Firestore document reference syntax

Which option correctly gets a reference to a document with ID 'doc1' inside the 'orders' collection?

A const docRef = db.collection('orders').doc('doc1');

B const docRef = db.doc('orders').collection('doc1');

C const docRef = db.collection('orders/doc1');

D const docRef = db.doc('orders/doc1');

💡 Hint

Remember the method to get a document inside a collection.

To get a document reference, use collection('name').doc('id'). Option A is valid syntax but less common; however, in Firebase SDK, db.doc('orders/doc1') is also valid. To avoid ambiguity, only option A is accepted here as it is the standard approach.

❓ optimization

advanced

Efficiently querying nested subcollections

You want to retrieve all documents from the 'comments' subcollection under multiple 'posts' documents. Which approach is most efficient?

A Use a join query between 'posts' and 'comments' collections.

B Store all comments in a single top-level collection instead of subcollections.

C Query each 'comments' subcollection under every 'posts' document separately.

D Use a collection group query on 'comments' to get all comments across posts.

💡 Hint

Firestore supports special queries to search across subcollections with the same name.

Collection group queries allow querying all subcollections with the same name across the database efficiently, avoiding multiple queries.

🔧 Debug

expert

Identify the error in nested document creation

What error will this code produce when trying to add a document to a subcollection?

Firebase

await db.collection('users').doc('user1').collection('orders').add({item: 'book', price: 10});

A No error; document is added successfully.

B ReferenceError: db is not defined.

C FirebaseError: Missing or insufficient permissions.

D TypeError: collection is not a function.

💡 Hint

Check if the code syntax is correct and if the database reference is valid.

The code correctly references a subcollection and adds a document. If permissions are set correctly and db is defined, no error occurs.