How to prevent duplicate documents in Firestore?

 

Step 1: Understand the Problem of Duplicates

Before we start implementing the solution, it's crucial to understand why duplicates might be created in Firestore. Duplicates can occur if the same data is unintentionally sent to Firestore multiple times. This could result due to network issues, retry logic, or bugs in your application logic. To prevent duplicates, you will typically use unique identifiers that can ensure a document is only written once into a specific Firestore collection.

 

Step 2: Define a Unique Field for Documents

The first step towards avoiding duplicates is identifying a field in your documents that should be unique. For example, if you have a collection of users, you might choose the email address as a unique identifier. Ensure you have this logic clearly defined as it'll be the key to preventing duplicates.

 

Step 3: Use Firestore Queries to Check for Existing Documents

Before adding a new document to your collection, perform a query to check if a document with the same unique field value already exists.

const checkForExistingDocument = async (firestore, collectionName, uniqueFieldValue) => {
  const querySnapshot = await firestore.collection(collectionName)
                                       .where('uniqueField', '==', uniqueFieldValue)
                                       .get();
  return !querySnapshot.empty;
};

If the query returns any documents, it means a document with that unique identifier already exists.

 

Step 4: Conditionally Add Document

Once you've confirmed that there are no existing documents with the same unique field value, you can safely add the new document.

const addDocumentIfNotExists = async (firestore, collectionName, data) => {
  const { uniqueField } = data;
  
  // Check if the document already exists
  const exists = await checkForExistingDocument(firestore, collectionName, uniqueField);
  
  if (!exists) {
    // Add the document since it doesn't exist
    await firestore.collection(collectionName).add(data);
    console.log('Document added successfully!');
  } else {
    console.log('Duplicate document exists, no new document was added.');
  }
};

This function first checks for an existing document and then adds the new document if it does not already exist.

 

Step 5: Implement Error Handling and Logic in Your Application

Ensure that your application has proper error handling to manage scenarios where document writing fails due to other reasons. Implement retry logic if necessary but ensure it respects the logic for uniqueness checks to prevent adding duplicates during retries.

try {
  await addDocumentIfNotExists(firestoreInstance, 'users', { uniqueField: '[email protected]', ... });
} catch (error) {
  console.error('Error adding document:', error.message);
  // Implement retry mechanism if needed but make sure to keep the uniqueness logic
}

 

Step 6: Consider Cloud Functions for Server-Side Checks

For added security and integrity, you might want to utilize Firebase Cloud Functions to ensure uniqueness on the server-side. This is especially useful if the client-side checks can be bypassed.

const admin = require('firebase-admin');

exports.checkAndAddUser = functions.https.onCall(async (data, context) => {
  const { email } = data;
  const firestore = admin.firestore();
  
  const exists = await checkForExistingDocument(firestore, 'users', email);
  
  if (!exists) {
    await firestore.collection('users').add(data);
    return { success: true, message: 'User added successfully!' };
  } else {
    return { success: false, message: 'User already exists!' };
  }
});