Firebase Firestore

Firebase Firestore Database

Status: Production Ready Last Updated: 2026-01-25 Dependencies: None (standalone skill) Latest Versions: firebase@12.8.0, firebase-admin@13.6.0

---

Quick Start (5 Minutes)

1. Install Firebase SDK

# Client SDK (web/mobile)
npm install firebase

# Admin SDK (server/backend)
npm install firebase-admin

2. Initialize Firebase (Client)

// src/lib/firebase.ts
import { initializeApp } from 'firebase/app';
import { getFirestore } from 'firebase/firestore';

const firebaseConfig = {
  apiKey: process.env.FIREBASE_API_KEY,
  authDomain: process.env.FIREBASE_AUTH_DOMAIN,
  projectId: process.env.FIREBASE_PROJECT_ID,
  storageBucket: process.env.FIREBASE_STORAGE_BUCKET,
  messagingSenderId: process.env.FIREBASE_MESSAGING_SENDER_ID,
  appId: process.env.FIREBASE_APP_ID,
};

const app = initializeApp(firebaseConfig);
export const db = getFirestore(app);

3. Initialize Firebase Admin (Server)

// src/lib/firebase-admin.ts
import { initializeApp, cert, getApps } from 'firebase-admin/app';
import { getFirestore } from 'firebase-admin/firestore';

// Initialize only once
if (!getApps().length) {
  initializeApp({
    credential: cert({
      projectId: process.env.FIREBASE_PROJECT_ID,
      clientEmail: process.env.FIREBASE_CLIENT_EMAIL,
      // Replace escaped newlines in private key
      privateKey: process.env.FIREBASE_PRIVATE_KEY?.replace(/\\n/g, '\n'),
    }),
  });
}

export const adminDb = getFirestore();

CRITICAL:

• Never expose FIREBASE_PRIVATE_KEY in client code
• Use Admin SDK for server-side operations (bypasses security rules)
• Use Client SDK for authenticated user operations

---

Core Operations

Document CRUD (Client SDK - Modular v9+)

import {
  collection,
  doc,
  addDoc,
  getDoc,
  getDocs,
  setDoc,
  updateDoc,
  deleteDoc,
  query,
  where,
  orderBy,
  limit,
  serverTimestamp,
  Timestamp,
} from 'firebase/firestore';
import { db } from './firebase';

// CREATE - Auto-generated ID
const docRef = await addDoc(collection(db, 'users'), {
  name: 'John Doe',
  email: 'john@example.com',
  createdAt: serverTimestamp(),
});
console.log('Created document with ID:', docRef.id);

// CREATE - Specific ID
await setDoc(doc(db, 'users', 'user-123'), {
  name: 'Jane Doe',
  email: 'jane@example.com',
  createdAt: serverTimestamp(),
});

// READ - Single document
const docSnap = await getDoc(doc(db, 'users', 'user-123'));
if (docSnap.exists()) {
  console.log('Document data:', docSnap.data());
} else {
  console.log('No such document!');
}

// READ - Collection with query
const q = query(
  collection(db, 'users'),
  where('email', '==', 'john@example.com'),
  orderBy('createdAt', 'desc'),
  limit(10)
);
const querySnapshot = await getDocs(q);
querySnapshot.forEach((doc) => {
  console.log(doc.id, ' => ', doc.data());
});

// UPDATE - Merge fields (doesn't overwrite entire document)
await updateDoc(doc(db, 'users', 'user-123'), {
  name: 'Jane Smith',
  updatedAt: serverTimestamp(),
});

// UPDATE - Set with merge (creates if doesn't exist)
await setDoc(doc(db, 'users', 'user-123'), {
  lastLogin: serverTimestamp(),
}, { merge: true });

// DELETE
await deleteDoc(doc(db, 'users', 'user-123'));

Document CRUD (Admin SDK)

import { adminDb } from './firebase-admin';
import { FieldValue, Timestamp } from 'firebase-admin/firestore';

// CREATE
const docRef = await adminDb.collection('users').add({
  name: 'John Doe',
  createdAt: FieldValue.serverTimestamp(),
});

// CREATE with specific ID
await adminDb.collection('users').doc('user-123').set({
  name: 'Jane Doe',
  createdAt: FieldValue.serverTimestamp(),
});

// READ
const doc = await adminDb.collection('users').doc('user-123').get();
if (doc.exists) {
  console.log('Document data:', doc.data());
}

// READ with query
const snapshot = await adminDb
  .collection('users')
  .where('email', '==', 'john@example.com')
  .orderBy('createdAt', 'desc')
  .limit(10)
  .get();

snapshot.forEach((doc) => {
  console.log(doc.id, '=>', doc.data());
});

// UPDATE
await adminDb.collection('users').doc('user-123').update({
  name: 'Jane Smith',
  updatedAt: FieldValue.serverTimestamp(),
});

// DELETE
await adminDb.collection('users').doc('user-123').delete();

---

Real-Time Listeners (Client SDK)

import { onSnapshot, query, where, collection, doc } from 'firebase/firestore';
import { db } from './firebase';

// Listen to single document
const unsubscribe = onSnapshot(doc(db, 'users', 'user-123'), (doc) => {
  if (doc.exists()) {
    console.log('Current data:', doc.data());
  }
});

// Listen to collection with query
const q = query(
  collection(db, 'messages'),
  where('roomId', '==', 'room-123'),
  orderBy('createdAt', 'desc'),
  limit(50)
);

const unsubscribeMessages = onSnapshot(q, (querySnapshot) => {
  const messages: Message[] = [];
  querySnapshot.forEach((doc) => {
    messages.push({ id: doc.id, ...doc.data() } as Message);
  });
  // Update UI with messages
  setMessages(messages);
});

// Handle errors
const unsubscribeWithError = onSnapshot(
  doc(db, 'users', 'user-123'),
  (doc) => {
    // Handle updates
  },
  (error) => {
    console.error('Listener error:', error);
    // Handle permission denied, etc.
  }
);

// IMPORTANT: Unsubscribe when done (React useEffect cleanup)
useEffect(() => {
  const unsubscribe = onSnapshot(/* ... */);
  return () => unsubscribe();
}, []);

CRITICAL:

• Always unsubscribe from listeners to prevent memory leaks
• Listeners count against your concurrent connection limit
• Each listener is a WebSocket connection

---

Query Patterns

Compound Queries

import { query, where, orderBy, limit, startAfter, collection } from 'firebase/firestore';

// Multiple where clauses (requires composite index)
const q = query(
  collection(db, 'products'),
  where('category', '==', 'electronics'),
  where('price', '=', new Date('2025-01-01')),
  where('date', ' ({ id: doc.id, ...doc.data() }));
}

Collection Group Queries

import { collectionGroup, query, where, getDocs } from 'firebase/firestore';

// Query across all subcollections named 'comments'
// Structure: posts/{postId}/comments/{commentId}
const q = query(
  collectionGroup(db, 'comments'),
  where('authorId', '==', 'user-123')
);

const snapshot = await getDocs(q);
// Returns all comments by user-123 across all posts

CRITICAL: Collection group queries require an index. Create in Firebase Console or deploy via firestore.indexes.json.

---

Batch Operations & Transactions

Batch Writes (Up to 500 operations)

import { writeBatch, doc, collection, serverTimestamp } from 'firebase/firestore';
import { db } from './firebase';

const batch = writeBatch(db);

// Add multiple documents
const usersRef = collection(db, 'users');
batch.set(doc(usersRef), { name: 'User 1', createdAt: serverTimestamp() });
batch.set(doc(usersRef), { name: 'User 2', createdAt: serverTimestamp() });

// Update existing document
batch.update(doc(db, 'counters', 'users'), { total: 100 });

// Delete document
batch.delete(doc(db, 'temp', 'old-doc'));

// Commit all operations atomically
await batch.commit();

Transactions (Read then Write)

import { runTransaction, doc, increment } from 'firebase/firestore';
import { db } from './firebase';

// Transfer credits between users
async function transferCredits(fromId: string, toId: string, amount: number) {
  await runTransaction(db, async (transaction) => {
    const fromRef = doc(db, 'users', fromId);
    const toRef = doc(db, 'users', toId);

    const fromDoc = await transaction.get(fromRef);
    const toDoc = await transaction.get(toRef);

    if (!fromDoc.exists() || !toDoc.exists()) {
      throw new Error('User not found');
    }

    const fromCredits = fromDoc.data().credits;
    if (fromCredits  {
  if (err.code === 'failed-precondition') {
    // Multiple tabs open, persistence can only be enabled in one tab
    console.warn('Persistence failed: multiple tabs open');
  } else if (err.code === 'unimplemented') {
    // Browser doesn't support persistence
    console.warn('Persistence not supported');
  }
});

Handle Offline State

import { onSnapshot, doc, SnapshotMetadata } from 'firebase/firestore';

onSnapshot(doc(db, 'users', 'user-123'), (doc) => {
  const source = doc.metadata.fromCache ? 'local cache' : 'server';
  console.log(`Data came from ${source}`);

  if (doc.metadata.hasPendingWrites) {
    console.log('Local changes pending sync');
  }
});

---

Data Modeling Best Practices

Denormalization for Read Performance

// Instead of joining users and posts...
// Store author info directly in post document

// posts/{postId}
{
  title: 'My Post',
  content: '...',
  authorId: 'user-123',
  // Denormalized author data for fast reads
  author: {
    name: 'John Doe',
    avatarUrl: 'https://...'
  },
  createdAt: Timestamp
}

Subcollections vs Root Collections

// Subcollections: Good for parent-child relationships
// posts/{postId}/comments/{commentId}
// - Easy to query all comments for a post
// - Deleting post doesn't auto-delete comments (use Cloud Functions)

// Root collections: Good for cross-cutting queries
// comments (with postId field)
// - Easy to query all comments by a user across posts
// - Requires manual data consistency

Counter Pattern (High-Write Scenarios)

// Direct increment (low traffic)
await updateDoc(doc(db, 'posts', postId), {
  viewCount: increment(1)
});

// Distributed counter (high traffic - 1000+ writes/sec)
// Use Cloud Functions to aggregate shard counts
// counters/{counterId}/shards/{shardId}

---

Error Handling

Common Errors and Solutions

| Error | Cause | Solution | |-------|-------|----------| | permission-denied | Security rules blocking access | Check rules, ensure user authenticated | | not-found | Document doesn't exist | Use exists() check before accessing data | | already-exists | Document with ID already exists | Use setDoc with merge or generate new ID | | resource-exhausted | Quota exceeded | Upgrade plan or optimize queries | | failed-precondition | Index missing for query | Create composite index (link in error) | | unavailable | Service temporarily unavailable | Implement retry with backoff | | invalid-argument | Invalid query combination | Check query constraints (see below) | | deadline-exceeded | Operation timeout | Reduce data size or paginate |

Query Constraints

// INVALID: Multiple inequality filters on different fields
query(collection(db, 'posts'),
  where('date', '>', startDate),
  where('likes', '>', 100)  // ERROR: Can't use inequality on second field
);

// VALID: Use range on one field, equality on others
query(collection(db, 'posts'),
  where('category', '==', 'tech'),
  where('date', '>', startDate)
);

// INVALID: orderBy field different from inequality field
query(collection(db, 'posts'),
  where('date', '>', startDate),
  orderBy('likes')  // ERROR: Must orderBy('date') first
);

// VALID: orderBy inequality field first
query(collection(db, 'posts'),
  where('date', '>', startDate),
  orderBy('date'),
  orderBy('likes')
);

---

Known Issues Prevention

This skill prevents 10 documented Firestore errors:

| Issue # | Error/Issue | Description | How to Avoid | Source | |---------|-------------|-------------|--------------|--------| | #1 | permission-denied | Security rules blocking operation | Test rules in Firebase Console emulator first | Common | | #2 | failed-precondition (index) | Composite index missing | Click error link to create index, or define in firestore.indexes.json | Common | | #3 | Invalid query combination | Multiple inequality filters | Use inequality on one field only, equality on others | Docs | | #4 | Memory leak from listeners | Not unsubscribing from onSnapshot | Always call unsubscribe in cleanup (useEffect return) | Common | | #5 | Offline persistence conflict | Multiple tabs with persistence | Use persistentMultipleTabManager() or handle error | Docs | | #6 | Transaction side effects | Side effects run multiple times | Never perform side effects inside runTransaction | Docs | | #7 | Batch limit exceeded | More than 500 operations | Split into multiple batches | Docs | | #8 | resource-exhausted | Quota limits hit | Implement pagination, reduce reads, use caching | Common | | #9 | Private key newline issue | \\n not converted in env var | Use .replace(/\\n/g, '\n') on private key | Common | | #10 | Collection group query fails | Missing collection group index | Create index with queryScope: COLLECTION_GROUP | Docs |

---

Firebase CLI Commands

# Initialize Firestore
firebase init firestore

# Start emulators
firebase emulators:start --only firestore

# Deploy rules and indexes
firebase deploy --only firestore

# Export data (for backup)
gcloud firestore export gs://your-bucket/backups/$(date +%Y%m%d)

# Import data
gcloud firestore import gs://your-bucket/backups/20250125

---

Package Versions (Verified 2026-01-25)

{
  "dependencies": {
    "firebase": "^12.8.0"
  },
  "devDependencies": {
    "firebase-admin": "^13.6.0"
  }
}

---

Official Documentation

• Firestore Overview: https://firebase.google.com/docs/firestore
• Get Started: https://firebase.google.com/docs/firestore/quickstart
• Data Model: https://firebase.google.com/docs/firestore/data-model
• Security Rules: https://firebase.google.com/docs/firestore/security/get-started
• Query Data: https://firebase.google.com/docs/firestore/query-data/queries
• Manage Data: https://firebase.google.com/docs/firestore/manage-data/add-data
• Offline Data: https://firebase.google.com/docs/firestore/manage-data/enable-offline

---

Last verified: 2026-01-25 | Skill version: 1.0.0

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Kgeminic
• Source: Kgeminic/claude-skills-1
• License: MIT

Install and usage instructions live in the source repository linked above.