Limit and pagination in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to limit the number of documents retrieved to 5.

Firebase

db.collection('users').limit([1]).get()

Drag options to blanks, or click blank then click option'

A15

B20

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number larger than needed.

Forgetting to use the limit method.

The limit method restricts the number of documents returned. Here, setting it to 5 means only 5 documents will be fetched.

2fill in blank

medium

Complete the code to start the query after the last document snapshot stored in 'lastDoc'.

Firebase

db.collection('users').startAfter([1]).limit(5).get()

Drag options to blanks, or click blank then click option'

AfirstDoc

BlastDoc

CprevDoc

DnextDoc

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable that does not hold a document snapshot.

Confusing 'startAfter' with 'startAt'.

The startAfter method is used to continue fetching documents after a specific document snapshot. Here, 'lastDoc' is the last document from the previous query.

3fill in blank

hard

Fix the error in the code to correctly paginate documents by starting at the last document snapshot.

Firebase

db.collection('users').[1](lastDoc).limit(5).get()

Drag options to blanks, or click blank then click option'

AstartAfter

BstartAt

CendBefore

DendAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'startAt' which causes duplicate documents in pagination.

Using 'endAt' or 'endBefore' which are for ending queries.

startAfter is the correct method to start fetching documents after the given document snapshot, enabling pagination without repeating the last document.

4fill in blank

hard

Fill both blanks to create a query that orders users by 'age' and limits results to 10.

Firebase

db.collection('users').orderBy([1]).[2](10).get()

Drag options to blanks, or click blank then click option'

A'age'

Blimit

CstartAfter

D'name'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'startAfter' instead of 'limit' for limiting results.

Ordering by a wrong field name.

The orderBy method sorts documents by the 'age' field. The limit method restricts the number of documents to 10.

5fill in blank

hard

Fill all three blanks to paginate users ordered by 'score', starting after 'lastDoc', limiting to 7 results.

Firebase

db.collection('users').orderBy([1]).[2]([3]).limit(7).get()

Drag options to blanks, or click blank then click option'

A'score'

BstartAfter

ClastDoc

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'limit' instead of 'startAfter' for pagination.

Not ordering by the correct field.

Passing a wrong variable to 'startAfter'.

This query orders users by 'score', starts after the last document snapshot 'lastDoc' to avoid duplicates, and limits the results to 7 documents.

Practice

1. What does the limit() function do in Firebase queries?

easy

A. It restricts the number of documents returned by the query.

B. It sorts the documents in ascending order.

C. It filters documents based on a condition.

D. It deletes documents from the collection.

Solution

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

   The limit() function is used to control how many documents are returned in a query result.

2. Step 2: Compare options with limit() behavior

   Only It restricts the number of documents returned by the query. correctly describes that limit() restricts the number of documents returned.

3. Final Answer:

   It restricts the number of documents returned by the query. -> Option A

4. Quick Check:

   limit() controls result count = B [OK]

Hint: Limit sets max items returned in query [OK]

Common Mistakes:

• Confusing limit() with orderBy()
• Thinking limit() filters data
• Assuming limit() deletes data

2. Which of the following is the correct syntax to get the first 5 documents ordered by 'timestamp' in Firebase?

easy

A. db.collection('posts').limit(5).orderBy('timestamp')

B. db.collection('posts').orderBy('timestamp').limit(5)

C. db.collection('posts').limit(5, 'timestamp')

D. db.collection('posts').orderBy('timestamp', 5)

Solution

1. Step 1: Recall correct method chaining order

   In Firebase, orderBy() must come before limit() to sort then limit results.

2. Step 2: Check each option's method order and parameters

   db.collection('posts').orderBy('timestamp').limit(5) correctly orders by 'timestamp' then limits to 5 documents. Others have wrong order or invalid parameters.

3. Final Answer:

   db.collection('posts').orderBy('timestamp').limit(5) -> Option B

4. Quick Check:

   orderBy before limit = C [OK]

Hint: Always orderBy before limit in Firebase queries [OK]

Common Mistakes:

• Placing limit() before orderBy()
• Passing limit a field name
• Using orderBy with a number parameter

3. Given this Firebase query code:

const firstPage = await db.collection('items').orderBy('name').limit(3).get();
const lastVisible = firstPage.docs[firstPage.docs.length - 1];
const nextPage = await db.collection('items').orderBy('name').startAfter(lastVisible).limit(3).get();
console.log(nextPage.docs.length);

What will be printed if the collection has 7 documents?

medium

A. 3

B. 4

C. 7

D. 0

Solution

1. Step 1: Analyze first page query

   The first query orders by 'name' and limits to 3 documents, so it gets documents 1 to 3.

2. Step 2: Analyze next page query

   The next query starts after the last document of the first page and limits to 3 documents. Since total is 7, remaining documents are 4 (docs 4 to 7), but limit(3) returns 3 documents.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   7 total - 3 first page = 4 remaining, limit(3) = 3 [OK]

Hint: Subtract first page count from total for next page size [OK]

Common Mistakes:

• Assuming next page always has same limit count
• Ignoring startAfter effect
• Counting all documents again

4. You wrote this Firebase pagination code but it throws an error:

const firstPage = await db.collection('users').limit(5).orderBy('age').get();

What is the error and how to fix it?

medium

A. Error: get() must be called before limit(); call get() first.

B. Error: limit() requires a field name; add 'age' as parameter.

C. Error: orderBy must come before limit; fix by swapping the methods.

D. No error; code works fine.

Solution

1. Step 1: Identify method order error

   Firebase requires orderBy() before limit() to sort data before limiting.

2. Step 2: Fix by swapping method calls

   Change code to db.collection('users').orderBy('age').limit(5).get() to fix the error.

3. Final Answer:

   Error: orderBy must come before limit; fix by swapping the methods. -> Option C

4. Quick Check:

   orderBy before limit fixes error = A [OK]

Hint: Always call orderBy before limit to avoid errors [OK]

Common Mistakes:

• Calling limit before orderBy
• Passing field name to limit
• Calling get() too early

5. You want to paginate through a Firebase collection with 12 documents, showing 5 per page. After fetching the first two pages, how many documents will the third page query return?

hard

A. 0

B. 3

C. 5

D. 2

Solution

1. Step 1: Calculate documents fetched in first two pages

   Each page shows 5 documents, so first two pages fetch 5 + 5 = 10 documents.

2. Step 2: Calculate remaining documents for third page

   Total documents are 12, so remaining are 12 - 10 = 2 documents.

3. Step 3: Consider limit on third page

   Limit is 5, but only 2 documents remain, so third page returns 2 documents.

4. Final Answer:

   2 -> Option D

5. Quick Check:

   12 - 10 = 2 documents left for page 3 [OK]

Hint: Subtract fetched docs from total to find last page size [OK]

Common Mistakes:

• Assuming all pages have full 5 documents
• Ignoring total document count
• Confusing page number with document count