Vfrdtyky

I'm developing a social media application. In that, I need to check whether the user already exists or not when he login with otp (like in WhatsApp).
I have found similar questions like this in stackoverflow, but none of the questions have correct answer. Help me!

You can just use the admin SDK and lookup the user by phone number:
https://firebase.google.com/docs/auth/admin/manage-users#retrieve_user_data

admin.auth().getUserByPhoneNumber(phoneNumber)

  .then(function(userRecord) {

    // User found.

  })

  .catch(function(error) {

    // User not found.

 });

If you try to create an user that already exists, Firebase will throw you an exception. You can handle that exception to do what you want.

Hope this helps!

First store the Firebase Phone Auth in FirebaseDatabase after every successful registration, then add a check when user try and enter the phone number if it already exist in database, If present then you got your answer otherwise register the user.
I hope the concept is clear. Will update you with the code soon.

after a research, I found this answer.

In firebase, each phone number have a unique user id. So, if a user signs up from different devices, or reinstalls the application with a same phone number, he/she will get the same user Id. So with that, we can check our database like this:

mAuth.signInWithCredential(credential).addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {

@Override

public void onComplete(@NonNull Task<AuthResult> task) {

if (task.isSuccessful()) {

final FirebaseUser user = task.getResult().getUser();

String uid = user.getUid();

final FirebaseFirestore db = FirebaseFirestore.getInstance();

final DocumentReference docRef = db.collection("users").document(uid);

docRef.get().addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {

    @Override

    public void onSuccess(final DocumentSnapshot documentSnapshot) {

        if (documentSnapshot.exists()) {

        //redirect to home page

        }

        else{

        //redirect to sign up page

        }

 }

Hope this will help someone :)

@Soorya. If you want to make your application like WhatsApp then you have to simply authenticate the user using phone number and OTP. When the user gets verified successfully then check whether user details are available in Firebase Database or FireStore. If available then simply redirect the user to home screen otherwise redirect the user to the registration page. For that refer below code snippet.

mFirebaseAuth

           .signInWithCredential(phoneAuthCredential)

           .addOnCompleteListener(new OnCompleteListener<AuthResult>() {

            public void onComplete(@NonNull Task<AuthResult> task) {

                if (task.isSuccessful()) {

                    goToNextActivity(((AuthResult) task.getResult()).getUser());

                    return;

                }

            }

        });



private void goToNextActivity(FirebaseUser firebaseUser) {

    FirebaseDatabase

            .getInstance()

            .getReference()

            .child("your table name")

            .child("user id details")

            .addListenerForSingleValueEvent(new ValueEventListener() {

                public void onDataChange(DataSnapshot dataSnapshot) {

                    if (dataSnapshot == null || dataSnapshot.getChildrenCount() <= 0) {

                        goToRegisterDetails(firebaseUser);

                        return;

                    }



                    User user = (User) dataSnapshot.getValue(User.class);

                    if (user == null 

                        || user.getUser_name() == null 

                        || user.getUser_name().isEmpty() 

                        || user.getUser_mobile() == null 

                        || user.getUser_mobile().isEmpty()) {

                        goToRegisterDetails(firebaseUser);

                    } else {

                        goToMainActivity(user);

                    }

                }



                public void onCancelled(DatabaseError databaseError) {

                    OTPVerificationActivity.this.hideProgressDialog();

                    Logger.e(databaseError.getMessage());

                }

            });

        }

I hope that you will get your answer. You can easily achieve that you want to do in your application.