Updating a single value in firebase Doc

May 10, 2022

first time using firebase, i’m creating a blog, when i create a post i’m creating it as:

  const postCollection = collection(database, "posts");

  const submitPost = async () => {
    await addDoc(postCollection, {
      title,
      body,
      comments: [],
      liked: false,
      user: { name: auth.currentUser.displayName, id: auth.currentUser.uid },
    });
  };

Below each post i have a comment section, but i’m a bit confused about adding the comments.

I tried this:

const addComment = async (id) => {
    const targetPost = doc(database, "posts", id);
    await setDoc(targetPost, { ...targetPost, comments: [comment] });
  };

But it didn’t work.
Thanks in advance

>Solution :

If a post already exists then you can use updateDoc() to update specific fields in that document instead of setDoc() that will overwrite the document if exists. Since ‘comments’ is an array, you can use arrayUnion() to push new comments to your posts as shown below:

import { doc, updateDoc, arrayUnion } from "firebase/firestore";

const addComment = async (id) => {
  const targetPost = doc(database, "posts", id);

  await updateDoc(targetPost, {
    comments: arrayUnion(comment)
  });
};

Do not that if you need to update a specific comment then you’ll have to read the whole post document, update comments array manually and then write the whole comments array back:

import { doc, getDoc, updateDoc } from "firebase/firestore";

const addComment = async (id) => {
  const targetPost = doc(database, "posts", id);
  
  const snapshot = await getDoc(targetPost)

  await updateDoc(targetPost, {
    comments: [...snapshot.data().comments, comment]
  });
};

Also checkout: Is there any way to update a specific index from the array in Firestore

For deleting a specific comment, you can use arrayRemove() if you know the exact comment object.