Subcollections in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a subcollection in Firebase?

A subcollection is a collection of documents nested inside a document. It helps organize data hierarchically, like folders inside folders.

beginner

How do you access a subcollection in Firebase?

You access a subcollection by first referencing its parent document, then calling the subcollection name on that document reference.

intermediate

Can subcollections contain their own subcollections?

Yes, subcollections can have their own subcollections, allowing deep nesting of data for complex structures.

intermediate

Why use subcollections instead of large flat collections?

Subcollections help keep data organized and improve performance by grouping related data together, reducing large queries on flat collections.

advanced

How does security work with subcollections in Firebase?

Security rules apply separately to subcollections, so you can control access to each subcollection independently based on your app's needs.

What is the first step to access a subcollection in Firebase?

A Reference the parent document

B Directly query the subcollection name

C Use a flat collection query

D Create a new collection

You must first reference the parent document before accessing its subcollection.

Can subcollections exist without a parent document?

A Only if they are empty

B Yes, they are independent

C No, they must be inside a document

D Only in test mode

Subcollections always belong to a parent document; they cannot exist alone.

What is a benefit of using subcollections?

A Makes data flat and simple

B Improves data organization

C Removes need for security rules

D Automatically indexes all data

Subcollections help organize data hierarchically, making it easier to manage.

How does Firebase security apply to subcollections?

A Security rules only apply to top-level collections

B Security rules are not needed for subcollections

C Subcollections inherit all rules from parent documents automatically

D Security rules apply separately to each subcollection

You can set specific security rules for each subcollection independently.

Which of these is true about nesting subcollections?

A Subcollections can have their own subcollections

B Nesting is limited to two levels

C Subcollections cannot be nested

D Nesting causes data loss

Firebase allows deep nesting of subcollections inside documents.

Explain how subcollections help organize data in Firebase.

Think about folders inside folders on your computer.

Describe how you would secure data in a subcollection differently from its parent document.

Security can be set separately for each part of your data.

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