Subcollections in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get a reference to a subcollection named 'orders' inside a document.

Firebase

const ordersRef = firestore.collection('users').doc('user123').[1]('orders');

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

Aadd

Bdoc

Ccollection

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using doc() instead of collection() to access a subcollection

Trying to use get() or add() directly to get a reference

To access a subcollection, use collection on a document reference.

2fill in blank

medium

Complete the code to add a new document to the 'orders' subcollection.

Firebase

firestore.collection('users').doc('user123').collection('orders').[1]({ item: 'book', quantity: 1 });

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

Aset

Bget

Cupdate

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using set() without a document reference

Using get() which only reads data

Use add() to create a new document with an auto-generated ID in a collection.

3fill in blank

hard

Fix the error in the code to correctly get a document from a subcollection.

Firebase

const orderDoc = firestore.collection('users').[1]('user123').collection('orders').doc('order456');

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

Aadd

Bdoc

Ccollection

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using collection() instead of doc() to get a document

Using get() which fetches data, not references

Use doc() to get a specific document by ID inside a collection.

4fill in blank

hard

Fill both blanks to query all documents in the 'comments' subcollection of a post.

Firebase

const commentsQuery = firestore.collection('posts').doc('post789').[1]('comments').[2]();

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

Acollection

Bdoc

Cget

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using doc() instead of collection() to access subcollection

Forgetting to call get() to fetch documents

First get the 'comments' subcollection, then call get() to fetch all documents.

5fill in blank

hard

Fill all three blanks to update a field in a document inside a subcollection.

Firebase

firestore.collection('[1]').doc('[2]').collection('[3]').doc('doc001').update({ status: 'completed' });

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

Aprojects

Bproj123

Ctasks

Dusers

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up collection and document names

Using wrong collection names for subcollections

Update a document in the 'tasks' subcollection of the 'proj123' document inside 'projects' collection.

Practice

1. What is a subcollection in Firebase Firestore?

easy

A. A backup copy of a collection stored in another region

B. A special type of document that stores user credentials

C. A collection nested inside a document to organize related data

D. A collection that automatically deletes old documents

Solution

1. Step 1: Understand Firestore data structure

   Firestore stores data in collections and documents. Documents can contain subcollections.

2. Step 2: Define subcollection purpose

   A subcollection is a collection inside a document used to organize related data hierarchically.

3. Final Answer:

   A collection nested inside a document to organize related data -> Option C

4. Quick Check:

   Subcollection = nested collection inside document [OK]

Hint: Subcollections are collections inside documents [OK]

Common Mistakes:

• Thinking subcollections are special documents
• Confusing subcollections with backups
• Assuming subcollections auto-delete data

2. Which of the following is the correct way to access a subcollection named comments inside a document with ID post1 in the posts collection?

easy

A. db.collection('comments').doc('post1').collection('posts')

B. db.collection('posts').collection('post1').doc('comments')

C. db.doc('posts/post1/comments')

D. db.collection('posts').doc('post1').collection('comments')

Solution

1. Step 1: Identify the correct chaining method

   To access a subcollection, first get the parent collection, then the document, then the subcollection.

2. Step 2: Match the correct syntax

   db.collection('posts').doc('post1').collection('comments') correctly chains collection('posts'), doc('post1'), then collection('comments').

3. Final Answer:

   db.collection('posts').doc('post1').collection('comments') -> Option D

4. Quick Check:

   Access subcollection by chaining collection, doc, collection [OK]

Hint: Chain collection, doc, then subcollection to access nested data [OK]

Common Mistakes:

• Swapping doc and collection order
• Using doc() instead of collection() for subcollections
• Trying to access subcollection directly without parent doc

3. Given the following code snippet, what will console.log(snapshot.docs.length) output if the comments subcollection has 3 documents?

const commentsRef = db.collection('posts').doc('post1').collection('comments');
const snapshot = await commentsRef.get();
console.log(snapshot.docs.length);

medium

A. 3

B. 1

C. 0

D. undefined

Solution

1. Step 1: Understand what get() returns

   The get() method on a collection reference returns a snapshot containing all documents in that collection.

2. Step 2: Count documents in the snapshot

   snapshot.docs.length gives the number of documents retrieved, which is 3 as stated.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Docs count in subcollection = 3 [OK]

Hint: snapshot.docs.length shows number of documents fetched [OK]

Common Mistakes:

• Assuming get() returns a single document
• Confusing docs.length with data length
• Expecting undefined if subcollection exists

4. You wrote this code to add a comment to a post's subcollection but it throws an error:

const commentRef = db.collection('posts').collection('post1').collection('comments').doc();
await commentRef.set({ text: 'Nice post!' });

What is the error in this code?

medium

A. Using collection('post1') instead of doc('post1') to access the post document

B. Missing await keyword before set()

C. doc() requires an ID argument

D. Cannot add documents to subcollections

Solution

1. Step 1: Check the chaining of collection and doc

   Subcollections must be accessed by first selecting the parent document with doc('post1'), not collection('post1').

2. Step 2: Identify the incorrect method call

   Using collection('post1') treats 'post1' as a collection, causing an error.

3. Final Answer:

   Using collection('post1') instead of doc('post1') to access the post document -> Option A

4. Quick Check:

   Access document with doc(), not collection() [OK]

Hint: Use doc() for documents, collection() for collections [OK]

Common Mistakes:

• Confusing collection() and doc() methods
• Assuming doc() always needs an ID
• Thinking subcollections can't have new documents

5. You want to model a blog where each post has multiple comments stored as a subcollection. Which approach is best to keep your Firestore database scalable and organized?

hard

A. Store all comments in a single top-level collection named 'comments' with a postId field

B. Store comments as a subcollection inside each post document

C. Store comments inside the post document as an array field

D. Create a separate Firestore database for comments

Solution

1. Step 1: Consider Firestore best practices for related data

   Storing comments as a subcollection inside each post keeps related data grouped and queries efficient.

2. Step 2: Evaluate other options

   Store all comments in a single top-level collection named 'comments' with a postId field can cause large collections and complex queries; Store comments inside the post document as an array field risks document size limits; Create a separate Firestore database for comments is unnecessary complexity.

3. Final Answer:

   Store comments as a subcollection inside each post document -> Option B

4. Quick Check:

   Subcollections keep related data organized and scalable [OK]

Hint: Use subcollections for related lists to keep data organized [OK]

Common Mistakes:

• Putting large arrays inside documents risking size limits
• Using one big collection causing slow queries
• Creating multiple databases unnecessarily