Subcollections in Firebase - Commands & Configuration

Introduction

Sometimes you want to organize your data in groups inside other groups. Subcollections let you keep related data inside a main group, like folders inside folders. This helps keep your data neat and easy to find.

When you want to store comments inside a specific blog post document.

When you need to keep user orders inside each user’s profile document.

When you want to group messages inside a chat room document.

When you want to separate settings for each project inside the project document.

When you want to keep logs or history inside a main record without mixing with other data.

Commands

This command creates a main document for a user with ID user123 in the users collection. It sets up the main group where subcollections can be added.

Terminal

firebase firestore:documents:create users/user123

Expected Output

Document users/user123 created successfully.

This command creates a subcollection document order001 inside the orders subcollection of the user123 document. It shows how to add a nested group inside the main group.

Terminal

firebase firestore:documents:create users/user123/orders/order001

Expected Output

Document users/user123/orders/order001 created successfully.

This command retrieves the subcollection document order001 inside the orders subcollection of user123. It verifies that the nested document exists and can be accessed.

Terminal

firebase firestore:documents:get users/user123/orders/order001

Expected Output

{

  "name": "projects/{projectId}/databases/(default)/documents/users/user123/orders/order001",

  "fields": {}

}

Key Concept

If you remember nothing else from this pattern, remember: subcollections let you organize data inside documents like folders inside folders for clear structure.

Common Mistakes

Trying to create a subcollection without first creating the parent document.

Subcollections must be inside an existing document; if the parent document does not exist, the subcollection cannot be created.

Always create the main document first before adding subcollections inside it.

Using the same collection name at different levels without clear structure.

This can cause confusion and make it hard to find data because collections with the same name at different levels look similar.

Use clear and distinct collection names to keep your data easy to navigate.

Summary

Create a main document first to hold your data group.

Add subcollections inside the main document to organize related data.

Use commands to create and verify documents and subcollections step-by-step.

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