Why does update() completely overwrites my data in Firebase? set() vs. update()

Question

I have went through Firebase doc, plus I have found a few topics on stackoverflow about set() vs. update() in Firebase: e.g.here

It is very clear what is the difference between the two of them.

In the following code, why does update() overwrites my existing data?

function saveChanges(event) {
    event.preventDefault();
    let modifiedTitle = document.getElementById('blog-title').value;
    let modifiedContent = document.getElementById('blog-content').value;
    let modifiedId = document.getElementById('blog-id-storage').innerHTML;
    let postData = 
        title: modifiedTitle,
        content: modifiedContent
    };
    let updates = {};
    updates[modifiedId] = postData;
    firebase.database().ref().child('posts/').update(updates);
}

I originally have a title, content, datePosted and Id and when I update it the title and content gets updated and dataPosted and Id gets deleted. Why? While this should be the behavior of set()?

Please edit the question to show the exact contents of `posts` before and after the update. — Doug Stevenson, Nov 24 '18 at 13:17

Doug Stevenson · Accepted Answer · 2018-11-24T18:04:52.430

7

The way update() works is that it only looks at the immediate children of the location where you called update. Everything underneath that location is replaced. So, what you're doing is replacing the entirety of postId-1 every time.

If you only really want to update the children of postId-1, then make that the base location where you call update():

firebase.database().ref().child('posts').child(modifiedId)
    .update(postDate)

edited Nov 24 '18 at 18:04

answered Nov 24 '18 at 13:40

Doug Stevenson

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here is the link to the documentation if you want to read more https://firebase.google.com/docs/database/admin/save-data#section-update – Reza Nasiri Nov 24 '18 at 16:27

Bakavani · Answer 2 · 2022-09-11T05:41:38.063

Following works for me:

admin.database().ref(SITE).child("ChildName").child("2ndLevelChild")
    .update({[KeyVariable]:valueVariable});

Aparently if I were to use it like the following it doesnt work (it overwrites:

            var newChild = {};
            newChild["ChildName/" + "2ndLevelChild"] = {
              [KeyVariable]: valueVariable
            };

           admin.database().ref(SITE).update(newChild);

Why does update() completely overwrites my data in Firebase? set() vs. update()

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