Polymer 2.0 + firebase can't update properly - firebase

Question

I've got a problem using Polymer 2.0 and Firebase.
I want to update data to Firebase with firebase-document but when I want to update only the title, it destroy all the previous data and save only the title.
Example of the strucute before update :
myapp:
categories:
1:
logoName: test.png
title: test
And after :
myapp:
categories:
1:
title: test bis
Do I have to give always the entire record and update only the field I want or can I only give the field I want to update to saveValue.
I try to only give the field but it doesn't seem to work
Here is a part of my code :
<dom-module id="categorie-form">
<template>
<firebase-document
id="document"
app-name="myapp"
data="{{categorieData}}">
</firebase-document>
<iron-form id="categorieIronForm">
<form id="categorieForm">
<label for="title">Nom de la catégorie</label>
<input type="text" name="title" id="title" value="[[name]]">
<paper-button id="validButton" on-click="_submitCategorie" raised>valider</paper-button>
</form>
</iron-form>
</template>
<script>
class CategorieForm extends Polymer.Element {
static get is () { return "categorie-form" }
static get properties () {
return {
categorieData: {
type: Object
}
}
}
_submitCategorie () {
this.categorieData = {
title: form.title.value
};
this.$.document.saveValue('/categories', key)
}
}
customElements.define(CategorieForm.is, CategorieForm);
</script>
</dom-module>
Thank you

Answer 1

try this :
this.$.document.saveValue('/categories/1/title', key)