Limit and pagination in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

Understanding Firestore Query Limit Behavior

You run a Firestore query with limit(5) on a collection with 10 documents. What will be the number of documents returned?

Firebase

const querySnapshot = await firestore.collection('users').limit(5).get();
const count = querySnapshot.size;

A 10

B 0

C 5

D Throws an error

💡 Hint

Think about what the limit function does in Firestore queries.

The limit function restricts the number of documents returned to the specified number, here 5, even if more documents exist.

❓ Architecture

intermediate

Choosing Pagination Strategy in Firestore

You want to paginate through a large Firestore collection efficiently. Which approach is best to avoid skipping or repeating documents?

A Use <code>limit()</code> with <code>offset()</code> to skip documents

B Use <code>limit()</code> with <code>startAfter()</code> using the last document snapshot

C Fetch all documents and slice the array in the client

D Use <code>limit()</code> with <code>where()</code> filtering by document ID

💡 Hint

Consider how Firestore handles offsets and document snapshots.

Using startAfter() with the last document snapshot is efficient and avoids skipping or repeating documents unlike offset() which can be costly.

❓ Configuration

advanced

Correct Firestore Query for Pagination with Limit

Which Firestore query correctly fetches the next 3 documents after a given document snapshot lastDoc?

Firebase

const lastDoc = await firestore.collection('items').orderBy('createdAt').limit(3).get().then(snap => snap.docs[2]);

A firestore.collection('items').limit(3).startAfter(lastDoc).get()

B firestore.collection('items').orderBy('createdAt').startAt(lastDoc).limit(3).get()

C firestore.collection('items').orderBy('createdAt').offset(3).limit(3).get()

D firestore.collection('items').orderBy('createdAt').startAfter(lastDoc).limit(3).get()

💡 Hint

Remember the difference between startAfter and startAt.

startAfter(lastDoc) skips the last document and fetches the next set. startAt includes the lastDoc again causing overlap.

❓ security

advanced

Securing Paginated Firestore Queries

You want to ensure users can only paginate through their own documents in Firestore. Which security rule correctly enforces this?

A match /items/{itemId} { allow read: if request.auth.uid == resource.data.ownerId; }

B match /items/{itemId} { allow read: if request.auth.uid != resource.data.ownerId; }

C match /items/{itemId} { allow read: if true; }

D match /items/{itemId} { allow read: if request.auth == null; }

💡 Hint

Think about how to restrict access to only the document owner.

The rule allows read only if the authenticated user's ID matches the document's ownerId field, securing pagination to user-owned docs.

🧠 Conceptual

expert

Impact of Using Offset in Firestore Pagination

What is the main drawback of using offset() for pagination in Firestore compared to startAfter()?

A Offset causes Firestore to read and discard skipped documents, increasing cost and latency

B Offset automatically caches documents, reducing read costs

C Offset guarantees no documents are skipped or repeated

D Offset is faster than startAfter because it uses indexes

💡 Hint

Consider how Firestore processes offset internally.

Offset makes Firestore read all skipped documents internally, which increases read operations and latency, unlike startAfter which uses document snapshots.