// <script src="https://www.gstatic.com/firebasejs/5.4.0/firebase.js"></script>

// Initialize Firebase
var project_id = "FIREBASE_PROJECT_ID";
var config = {
  apiKey: "API_KEY",
  authDomain: project_id + ".firebaseapp.com",
  databaseURL: "https://" + project_id + ".firebaseio.com",
  projectId: project_id,
  storageBucket: project_id + ".appspot.com",
  messagingSenderId: "CLOUD_MESSENGING_SENDER_ID"
};
firebase.initializeApp(config);

var db = firebase.database();
// Need to modify database rule for read and write permissions before going futher
// {
//   "rules": {
//     ".read": true,
//     ".write": true,
//     "products": {
//       ".indexOn": ["id"]
//     },
//     "categories": {
//       ".indexOn": ["id"]
//     }
//   }
// }

// EXAMPLE DB
// my-db
//   categories
//     0
//       "id": 1
//       "name: "Metal"
//     1
//       "id": 2
//       "name: "Plastic"
//   products
//     0
//       "id": 1
//       "name": "Chair"
//       "category: 2
//     1
//       "id": 2
//       "name": "Table"
//       "category: 1

// READ using on and once
// on(eventType, callback, cancelCallbackOrContext, context)
// eventType: "value", "child_added", "child_changed", "child_removed", or "child_moved"

var categoriesRef = db.ref('categories');

// Handle for new value:
categoriesRef.on('value', function(dataSnapshot) {
  //...
});

// Handle for new child added
categoriesRef.on('child_added', function(childSnapshot, prevChildKey) {
  //...
});

// Handle child removal:
categoriesRef.on('child_removed', function(oldChildSnapshot) {
  //...
});

// Handle child data changes:
categoriesRef.on('child_changed', function(childSnapshot, prevChildKey) {
  //...
});

// Handle child ordering changes:
categoriesRef.on('child_moved', function(childSnapshot, prevChildKey) {
  //...
});

// once(eventType, successCallback, failureCallbackOrContext, context)
// Read data once and won't read again when data has changed.
categoriesRef.once('value').then(function(dataSnapshot) {
  // handle read data.
});

// CREATE
// using set(value, onComplete)
// Beware of overwrite or delete the record with same KEY 
var newCatId = db.ref().child('categories').push().key; // auto generate new UID key
db.ref('categories/' + newCatId).set({
  id: 3,
  name: "Other"
});

// using push(value, onComplete) will return its reference
// Create a new category reference with an auto-generated id
var newCategoryRef = categoriesRef.push();
newCategoryRef.set({
  id: 4,
  name: "Something"
});

// UPDATE
var records = {};
records['categories/1'] = {name: "Wood"};
records['products/0'] = {category: 2};
// Could update multiple values
db.ref().update(records);
// This action will update the name of category with key 1 and update the category of product with key 0.

// DELETE
db.ref('categories/2').remove()
// This action will remove the category with key 2

db.ref('categories/2').set(null);
// Set null value to a key will delete record. It will do the same as remove() function.

// FIND
// https://firebase.google.com/docs/reference/js/firebase.database.Query
// https://firebase.google.com/docs/database/web/lists-of-data

var productsRef = db.ref('products');
// Find 10 latest products
productsRef.limitToLast(10);

// You can only use one order-by method at a time.
// Calling an order-by method multiple times in the same query throws an error.
productsRef.orderByChild('id'); // data will present in order of null, false, true (key in alphabetical order), number, text, object
productsRef.orderByKey();
productsRef.orderByValue();


productsRef.orderByKey().limitToFirst(10);
productsRef.orderByKey().limitToLast(10);

productsRef.orderByChild('id').startAt(2).endAt(5); // get product records with ID in [2:5]
productsRef.orderByChild('id').equalTo(1); // get product with ID equals to 1

// STORE DATA OFFLINE
// Detecting Connection State
var connectedRef = firebase.database().ref(".info/connected");
connectedRef.on("value", function(snap) {
  console.log((snap.val() !== true ? "not " : "") + "connected");
});

// Server Time
var userLastOnlineRef = firebase.database().ref("users/name/lastOnline");
userLastOnlineRef.onDisconnect().set(firebase.database.ServerValue.TIMESTAMP);

// Server time offset
var offsetRef = firebase.database().ref(".info/serverTimeOffset");
offsetRef.on("value", function(snap) {
  var offset = snap.val();
  var estimatedServerTimeMs = new Date().getTime() + offset;
});