firebase-cloud-firestore

name	firebase-cloud-firestore
description	Use when setting up Firestore, designing schemas, doing CRUD, creating listeners, paginating queries, configuring indexes, enabling offline persistence, or writing security rules.
license	MIT

Firebase Cloud Firestore Skill

This skill defines how to correctly implement Cloud Firestore in Flutter applications, covering data modeling, queries, real-time updates, security rules, and scale optimization.

When to Use

Use this skill when:

• Setting up and configuring Cloud Firestore in a Flutter project.
• Designing document and collection structure or planning subcollections.
• Performing read, write, batch, or transaction operations.
• Implementing real-time listeners or paginated queries.
• Optimizing for scale and avoiding write hotspots.
• Writing or debugging Firestore security rules.

---

1. Database Selection

Choose Cloud Firestore when the app needs:

• Rich, hierarchical data models with subcollections.
• Complex queries: chaining filters, combining filtering and sorting on a property.
• Transactions that atomically read and write data from any part of the database.
• High availability (typical uptime 99.999%) or critical-level reliability.
• Automatic scaling to millions of concurrent users.

Use Realtime Database instead for simple data models requiring simple lookups and extremely low-latency synchronization (typical response times under 10ms).

---

2. Setup and Configuration

flutter pub add cloud_firestore

import 'package:cloud_firestore/cloud_firestore.dart';

final db = FirebaseFirestore.instance; // after Firebase.initializeApp()

Location:

• Select the database location closest to users and compute resources.
• Use multi-region locations for critical apps (maximum availability and durability).
• Use regional locations for lower costs and lower write latency.

iOS/macOS: Consider pre-compiled frameworks to improve build times:

pod 'FirebaseFirestore',
  :git => 'https://github.com/invertase/firestore-ios-sdk-frameworks.git',
  :tag => 'IOS_SDK_VERSION'

Offline persistence is enabled by default on mobile. Configure cache size:

FirebaseFirestore.instance.settings = const Settings(
  persistenceEnabled: true,
  cacheSizeBytes: Settings.CACHE_SIZE_UNLIMITED,
);

---

3. Document Structure

• Avoid document IDs . and .. (special meaning in Firestore paths).
• Avoid forward slashes (/) in document IDs (path separators).
• Do not use monotonically increasing document IDs (e.g., Customer1, Customer2) — causes write hotspots.
• Use Firestore's automatic document IDs when possible:

final docRef = await db.collection("users").add({
  'name': 'Ada Lovelace',
  'email': '[e-mail]',
  'created_at': FieldValue.serverTimestamp(),
});
print('Created document with ID: ${docRef.id}');

• Avoid these characters in field names (require extra escaping): . [ ] * `
• Use subcollections within documents to organize complex, hierarchical data rather than deeply nested objects.

---

4. Indexing

• Firestore queries are indexed by default; query performance is proportional to the result set size, not the dataset size.
• Set collection-level index exemptions to reduce write latency and storage costs.
• Disable Descending and Array indexing for fields that do not need them.
• Exempt string fields with long values that are not used for querying.
• Exempt fields with sequential values (e.g., timestamps) from indexing if not used in queries — avoids the 500 writes/second index limit.
• Add single-field exemptions for TTL fields.
• Exempt large array or map fields not used in queries — avoids the 40,000 index entries per document limit.

---

5. Read and Write Operations

Read All Documents in a Collection

final querySnapshot = await db.collection("users").get();
for (var doc in querySnapshot.docs) {
  print("${doc.id} => ${doc.data()}");
}

Query with Filters

final query = db.collection("users")
    .where("age", isGreaterThanOrEqualTo: 18)
    .orderBy("age")
    .limit(20);

final results = await query.get();

Cursor-Based Pagination

// First page
final first = db.collection("cities").orderBy("name").limit(25);
final firstSnapshot = await first.get();

// Next page using last document as cursor
final lastDoc = firstSnapshot.docs.last;
final next = db.collection("cities")
    .orderBy("name")
    .startAfterDocument(lastDoc)
    .limit(25);

• Do not use offsets for pagination — use cursors to avoid retrieving and being billed for skipped documents.

Write with Server Timestamp

await db.collection("users").doc("user_1").set({
  'name': 'Grace Hopper',
  'updated_at': FieldValue.serverTimestamp(),
});

Batch Write (Atomic, Up to 500 Operations)

final batch = db.batch();
batch.set(db.collection("cities").doc("LA"), {'name': 'Los Angeles'});
batch.update(db.collection("cities").doc("SF"), {'population': 860000});
batch.delete(db.collection("cities").doc("OLD"));
await batch.commit();

Transaction

await db.runTransaction((transaction) async {
  final snapshot = await transaction.get(db.collection("counters").doc("visits"));
  final currentCount = snapshot.get("count") as int;
  transaction.update(snapshot.reference, {"count": currentCount + 1});
});

• Execute independent operations (e.g., a document lookup and a query) in parallel, not sequentially.
• Be aware of write rate limits: ~1 write per second per document.
• For writing a large number of documents, use a bulk writer instead of the atomic batch writer.

---

6. Designing for Scale

• Avoid high read or write rates to lexicographically close documents (hotspotting).
• Avoid creating new documents with monotonically increasing fields (like timestamps) at a very high rate.
• Avoid deleting documents in a collection at a high rate.
• Gradually increase traffic when writing to the database at a high rate — ramp up over 5 minutes.
• Avoid queries that skip over recently deleted data — use start_at to find the correct start point.
• Distribute writes across different document paths to avoid contention.
• Firestore scales automatically to ~1 million concurrent connections and 10,000 writes/second.

---

7. Real-time Updates

final subscription = db.collection("messages")
    .where("room", isEqualTo: "general")
    .orderBy("timestamp", descending: true)
    .limit(50)
    .snapshots()
    .listen((querySnapshot) {
      for (var change in querySnapshot.docChanges) {
        switch (change.type) {
          case DocumentChangeType.added:
            print("New message: ${change.doc.data()}");
            break;
          case DocumentChangeType.modified:
            print("Modified: ${change.doc.data()}");
            break;
          case DocumentChangeType.removed:
            print("Removed: ${change.doc.id}");
            break;
        }
      }
    });

// Detach when no longer needed:
subscription.cancel();

• Limit the number of simultaneous real-time listeners.
• Detach listeners when they are no longer needed to avoid memory leaks and unnecessary reads.
• Use compound queries to filter data server-side rather than filtering on the client.
• For large collections, use queries to limit the data being listened to — never listen to an entire large collection.

---

8. Security

• Always use Firebase Security Rules to protect Firestore data.
• Security rules do not cascade unless a wildcard is used.
• If a query's results might contain data the user does not have access to, the entire query fails.

Example rules for user-owned documents:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, update, delete: if request.auth != null && request.auth.uid == userId;
      allow create: if request.auth != null;
    }
  }
}

• Validate user input before submitting to Firestore to prevent injection attacks.
• Use transactions for operations that require atomic updates to multiple documents.
• Implement proper error handling for all Firestore operations.
• Never store sensitive information in Firestore without proper access controls.

---

References

• Cloud Firestore Flutter documentation
• Cloud Firestore best practices
• Cloud Firestore security rules