Pfthb

Let's say I have a collection named people, and each document of has a property name phone_number.

Of course, I could allow a user to register, and if his phone number exists, I may consider him validated. But if the user doesn't have a phone number in any of the documents I mentioned before, I wouldn't want to let him register from the start.

How can I tell Firebase to allow users to register only if their phone numbers exists in the documents? In a different backend (such as nest) I would create a pipe that simply checks if the given phone number in the request exists.

If I understand correctly your requirement, you should just check if there is a people doc with this phone_number in your collection, with a query through the Admin SDK.

So if I make the assumption you are going to use an HTTPS Cloud Function, something along the following lines should do the trick. It should be easy to adapt it to other Cloud Function triggers.

exports.registerUser = functions.https.onRequest((req, res) => 
 const phone_number = req.body.phone_number;
 const peopleCollecRef = admin.firestore().collection('people');

 peopleCollecRef
 .where('phone_number', '==', phone_number)
 .get()
 .then(querySnapshot => 
 if (querySnapshot.size === 1) 
 return admin.auth().createUser(
 email: req.body.email,
 emailVerified: false,
 phoneNumber: phone_number,
 password: '....'
 );
 else 
 throw new Error('No user (or more than one user) with corresponding phone number ');
 
 )
 .then(userRecord => 
 res.send( result: 'user created with id: ' + userRecord.uid ); //Just an example
 )
 .catch(error => 
 res.status(500).send(error);
 );
);

You should also probably flag when a user has been registered in order to avoid the phone number is used another time by another user. I men depending on your data model there are probably several checks to be implemented inthe Cloud Function.