How to push an array value in Firebase Firestore

Question

I am trying to push an array element but am destroying all the content there and replacing with the pushed data:

db .collection('households')
  .doc(householdId)
  .set( { users: [uid], }, { merge: true }, )
  .then(() => { resolve(); })
  .catch(() => reject());

I thought the merge true doesn't destroy the data that is already there? Struggling a little with the firestore api docs.

This is the structure of my data:

households
  2435djgnfk 
    users [ 
      0: user1 
      1: user2 
    ]

Thank you!

Answer 1

You should use Firestore Transaction for this.

const householdRef = db.collection('households').doc(householdId);

const newUid = '1234'; // whatever the uid is...

return db.runTransaction((t) => {
  return t.get(householdRef).then((doc) => {
    // doc doesn't exist; can't update
    if (!doc.exists) return;
    // update the users array after getting it from Firestore.
    const newUserArray = doc.get('users').push(newUid);
    t.set(householdRef, { users: newUserArray }, { merge: true });
  });
}).catch(console.log);

Updating an array or a stored object without getting it first will always destroy the older values inside that array/object in firestore.

This is because they are fields and not actual document themselves. So, you have to first get the document and then update the value after that.