Firebase cloud Function to read data change on one child. Join data from two childs and write to third child

Cloud function

//Read data change on your ref
exports.functionname = functions.database.ref('/Items/{itemid}')
    .onWrite(event => {
        const item = event.data.val();
        // read data once from second node
        const locationRef = event.data.adminRef.root.child('location').child(event.params.itemid);
        locationRef.once('value').then(snapshot =>{
            const location = snapshot.val();
            const city = location.city;
            // data node where you want to write data
            const CityRef = event.data.adminRef.root.child(city);
                return CityRef.set({
                    id: item.id,
                    name: item.name,
                    imageUrl: item.imageUrl,
                    price: item.price,
                    type: item.type,
                    latitude: location.latitude,
                    longitude: location.longitude
                });
        });  
    });