Collection group queries in Firebase - Mini Project: Build & Apply

Using Collection Group Queries in Firebase

📖 Scenario: You are building a simple app to track user comments on different posts. Each post has its own collection of comments stored in subcollections. You want to find all comments across all posts that contain a specific keyword.

🎯 Goal: Build a Firebase query that uses a collection group query to find all comments containing the word "great" in their text, regardless of which post they belong to.

📋 What You'll Learn

Create a Firestore collection called posts with at least two documents.

Each posts document should have a subcollection called comments with comment documents.

Create a variable called keyword set to the string "great".

Write a collection group query on comments to find all comments where the text field contains the keyword.

Store the query in a variable called query.

💡 Why This Matters

🌍 Real World

Collection group queries let you search across many subcollections with the same name, useful for apps with nested data like comments on posts or reviews on products.

💼 Career

Understanding collection group queries is important for building scalable, efficient Firestore apps that need to search data across multiple nested collections.

1

Set up Firestore data structure

Create a Firestore collection called posts with two documents having IDs post1 and post2. Each post document should have a subcollection called comments with two comment documents. The comments should have a text field with these exact values:

- For post1/comments: comment1 with text "This is great!", comment2 with text "Nice work"
- For post2/comments: comment1 with text "Great job", comment2 with text "Needs improvement"

Firebase

// Your code here to create posts collection with comments subcollections

Need a hint?

Use firestore.collection('posts') to get the posts collection. Then use doc() and collection('comments') to add comments.

2

Create a keyword variable

Create a variable called keyword and set it to the string "great".

Firebase

// Firestore data setup (example using Firebase Admin SDK or client SDK)
const posts = firestore.collection('posts');

// Add post1 with comments
posts.doc('post1').collection('comments').doc('comment1').set({ text: 'This is great!' });
posts.doc('post1').collection('comments').doc('comment2').set({ text: 'Nice work' });

// Add post2 with comments
posts.doc('post2').collection('comments').doc('comment1').set({ text: 'Great job' });
posts.doc('post2').collection('comments').doc('comment2').set({ text: 'Needs improvement' });

// Your code here to create keyword variable

Need a hint?

Use const keyword = 'great'; to create the variable.

3

Write the collection group query

Write a collection group query on comments to find all documents where the text field contains the keyword. Store this query in a variable called query. Use firestore.collectionGroup('comments') and a where clause with array-contains or >= and <= operators to filter text containing the keyword.

Firebase

// Firestore data setup (example using Firebase Admin SDK or client SDK)
const posts = firestore.collection('posts');

// Add post1 with comments
posts.doc('post1').collection('comments').doc('comment1').set({ text: 'This is great!' });
posts.doc('post1').collection('comments').doc('comment2').set({ text: 'Nice work' });

// Add post2 with comments
posts.doc('post2').collection('comments').doc('comment1').set({ text: 'Great job' });
posts.doc('post2').collection('comments').doc('comment2').set({ text: 'Needs improvement' });

const keyword = 'great';

// Your code here to create collection group query

Need a hint?

Use firestore.collectionGroup('comments') and chain two where clauses with >= and <= to filter text starting with the keyword.

4

Complete the query setup

Add a line to execute the query and store the results in a variable called results. Use await query.get() to fetch the documents.

Firebase

// Firestore data setup (example using Firebase Admin SDK or client SDK)
const posts = firestore.collection('posts');

// Add post1 with comments
posts.doc('post1').collection('comments').doc('comment1').set({ text: 'This is great!' });
posts.doc('post1').collection('comments').doc('comment2').set({ text: 'Nice work' });

// Add post2 with comments
posts.doc('post2').collection('comments').doc('comment1').set({ text: 'Great job' });
posts.doc('post2').collection('comments').doc('comment2').set({ text: 'Needs improvement' });

const keyword = 'great';

const query = firestore.collectionGroup('comments').where('text', '>=', keyword).where('text', '<=', keyword + '\uf8ff');

// Your code here to get query results

Need a hint?

Use const results = await query.get(); to fetch the matching documents.