如何将Firestore数据字段分配给变量？

Question

I have a firebase collection 'messages', and I add documents into it:

this.afs.collection('messages').add({ 'qn': message, 'student': this.student, 'upvotes': this.upvote });

The upvotes field is set by a upvote or downvote button. When either of them are clicked, I call this method:

increment(messageId) { //decrement(messageId) for the downvote button
this.upvote += 1; //-=1 for the downvote button   
this.messageDoc = this.afs.doc('messages/' + messageId);
this.message = this.messageDoc.valueChanges();
this.afs.doc('messages/' + messageId).update({
  upvotes: this.upvote
});
}

The problem with this is, I defined the upvotes variable as: private upvote: number = 0;

This means that if I reload the page and click upvote or downvote, the value will just increase or decrease starting from 0 again, as the variable upvote is not assigned the actual database value. I want to fix this by assigning the data in the upvotes field of the document into the upvote variable - how can I do this?

Edit: Managed to make it work using this

increment(messageId) {
this.upvote += 1;
let self = this;
let messageDoc = firebase.firestore().collection('messages').doc(messageId);

return firebase.firestore().runTransaction(function (transaction) {
  return transaction.get(messageDoc).then(function (sfDoc) {
    let upVote = sfDoc.data().upvotes + self.upvote;
    transaction.update(messageDoc, { upvotes: upVote });
    self.upvote = 0;
  });
}).then(function() {
  console.log("Transaction successfully committed!");
}).catch(function(err) {
  console.log("Transaction failed: " + err);
});
}

Answer 1

You can use Transactions for that

let self = this;
let messageDoc = firebase.firestore().collection('messages').doc(messageId);

return firebase.firestore().runTransaction((transaction) => {
    // This code may get re-run multiple times if there are conflicts.
    return transaction.get(messageDoc).then((sfDoc) => {
        let upVote = sfDoc.data().upvotes + self.upvote;
        transaction.update(messageDoc, { upvotes: upVote });
    });
  }).then(() => {
      console.log("Transaction successfully committed!");
  }).catch((err) => {
      console.log("Transaction failed: ", err);
  });

and don't forget to import import * as firebase from 'firebase';