How to Use Custom Metadata Storage in Firebase

To use custom metadata storage in Firebase, store your metadata as fields in Firestore documents or Realtime Database nodes. You create a structured object with your custom data and save it under a specific path, then read or update it as needed using Firebase SDK methods.

Syntax

In Firebase, custom metadata is stored as key-value pairs inside documents (Firestore) or nodes (Realtime Database). You define a path, then use set() or update() to save your metadata object.

• collection('yourCollection'): selects the collection
• doc('yourDocId'): selects the document
• set(data): writes data, replacing existing
• update(data): updates specific fields

javascript

const metadata = {
  createdBy: 'user123',
  createdAt: new Date().toISOString(),
  tags: ['firebase', 'metadata'],
  customField: 'customValue'
};

// Firestore syntax
firestore.collection('items').doc('item1').set(metadata);

// Realtime Database syntax
realtimeDb.ref('items/item1/metadata').set(metadata);

Example

This example shows how to save and read custom metadata for an item in Firestore using Firebase JavaScript SDK.

javascript

import { initializeApp } from 'firebase/app';
import { getFirestore, doc, setDoc, getDoc } from 'firebase/firestore';

const firebaseConfig = {
  apiKey: 'YOUR_API_KEY',
  authDomain: 'YOUR_AUTH_DOMAIN',
  projectId: 'YOUR_PROJECT_ID'
};

const app = initializeApp(firebaseConfig);
const firestore = getFirestore(app);

async function saveMetadata() {
  const metadata = {
    createdBy: 'user123',
    createdAt: new Date().toISOString(),
    tags: ['firebase', 'metadata'],
    customField: 'customValue'
  };

  await setDoc(doc(firestore, 'items', 'item1'), metadata);
  console.log('Metadata saved');
}

async function readMetadata() {
  const docRef = doc(firestore, 'items', 'item1');
  const docSnap = await getDoc(docRef);

  if (docSnap.exists()) {
    console.log('Metadata:', docSnap.data());
  } else {
    console.log('No metadata found');
  }
}

saveMetadata().then(() => readMetadata());

Output

Metadata saved
Metadata: { createdBy: 'user123', createdAt: '2024-06-01T12:00:00.000Z', tags: ['firebase', 'metadata'], customField: 'customValue' }

Common Pitfalls

Common mistakes when using custom metadata storage in Firebase include:

• Overwriting existing data unintentionally by using set() without { merge: true }.
• Not handling asynchronous calls properly, leading to race conditions.
• Storing large or complex objects that exceed Firestore document size limits.
• Not validating or sanitizing metadata before saving.

javascript

/* Wrong: overwrites entire document */
firestore.collection('items').doc('item1').set({ newField: 'value' });

/* Right: merges new field with existing data */
firestore.collection('items').doc('item1').set({ newField: 'value' }, { merge: true });

Quick Reference

Action	Method	Description
Save metadata	set(docRef, data)	Writes data to a document, replaces existing unless merged
Update metadata	update(docRef, data)	Updates specific fields without overwriting whole document
Read metadata	get(docRef)	Retrieves document data including metadata
Merge data	set(docRef, data, { merge: true })	Merges new data with existing document fields

Key Takeaways

Store custom metadata as fields inside Firestore documents or Realtime Database nodes.

Use set() with { merge: true } to avoid overwriting existing data unintentionally.

Always handle asynchronous calls with async/await or promises to ensure data consistency.

Keep metadata size within Firestore limits and validate data before saving.

Read metadata using get() to access stored custom fields easily.