Firestore query "onSnapshot" called at the same time does not work (

Question

I created an app with Ionic and Firestore that features live chat and I'm having a problem with it.

The conversation is loaded with the method:

refUneConversationMyUserCol.ref.orderBy('date', 'desc').limit(20).get()

To this is added an "onSnapshot" request to retrieve the last message sent live

 this.unsubscribeDataUneConversation = refUneConversationMyUserCol.ref.orderBy('date', 'desc').limit(1).onSnapshot(result => {
    console.log(result.docs[0].data());
    if (this.isCalledBySnapshot === false) {
      this.isCalledBySnapshot = true;
    } else if (result.docs[0].data().expediteur !== this.authentificationService.uidUserActif) {
      const data = result.docs[0].data();
      const id = result.docs[0].id;
      this.dataUneConversation.push({ id, ...data } as UneConversation);
    }
  });

It will work perfectly however, when I send a message at the same time (with 2 different accounts talking to each other), I encounter a problem, the onSnapshot is triggered only once and I only receive one message.

I specify that the two messages are sent well in the database, they are only not displayed both during the live session

Do you have any idea why?

Thank you

(Here is the whole method)

  async getDataUneConversation(idI: string) {
if (this.loadedDataUneConversation !== idI) {
  /* ANCHOR Msg en direct */
  this.isCalledBySnapshot = false;
  if (this.unsubscribeDataUneConversation) {
    await this.unsubscribeDataUneConversation();
  }

  const refUneConversationMyUserCol = this.afs.collection<User>('users').doc<User>(this.authentificationService.uidUserActif).collection<Conversations>('conversations');
  const result = await refUneConversationMyUserCol.ref.orderBy('date', 'desc').limit(20).get();

  /* ANCHOR Msg en direct */
  this.unsubscribeDataUneConversation = refUneConversationMyUserCol.ref.orderBy('date', 'desc').limit(1).onSnapshot(result => {
    console.log(result.docs[0].data());
    if (this.isCalledBySnapshot === false) {
      this.isCalledBySnapshot = true;
    } else if (result.docs[0].data().expediteur !== this.authentificationService.uidUserActif) {
      const data = result.docs[0].data();
      const id = result.docs[0].id;
      this.dataUneConversation.push({ id, ...data } as UneConversation);
    }
  });

  /* ANCHOR Msg en brut */
  if (result.docs.length < 20) {
    this.infiniteLastUneConversationMax = true;
  } else {
    this.infiniteLastUneConversationMax = false;
  }
  this.infiniteLastUneConversation = result.docs[result.docs.length - 1];
  this.dataUneConversation = result.docs.map(doc => {
    const data = doc.data();
    const id = doc.id;
    return { id, ...data } as UneConversation;
  });
  this.dataUneConversation.reverse();
  this.loadedDataUneConversation = idI;
}

}

EDIT for working :

    this.unsubscribeDataUneConversation = refUneConversationMyUserCol.ref.orderBy('date', 'asc').startAfter(this.dataUneConversation[this.dataUneConversation.length
- 1].date).onSnapshot(result => {
            result.docs.forEach(element => {
              const data = element.data();
              const id = element.id;
              if (!this.dataUneConversation.some(e => e.id === element.id)) {
                this.dataUneConversation.push({ id, ...data } as UneConversation);
              }
            });
          });

Answer 1

You're limiting live messages to only one last message. In a chat app, you want to listen to all new messages. So the issue is probably in your .limit(1) clause.

But if you do that, I understand that you'll get the whole conversation, with all messages, since the conversation started.

My approach would be like this:

1. Get the date of the last message from your refUneConversationMyUserCol... conversation loader.
2. When you do the onSnapshot() to get the last message, do not limit to 1 message, instead, start at a date after the date of the last loaded message.

Since you're ordering by date anyway, this will be an easy fix. Look into "Adding a cursor to your query".

Basically, you'll be saying to Firestore: give me LIVE new messages but start at NOW - and even if there are many messages posted at the same time, you'll get them all, since you're not limiting to 1.

Feel free to ask if this is not clear enough.