Retrieving JSON Data

JSON values are stored in database columns of type JSON. The data retrieval mechanism for JSON values is the same as for any other data stored in Backendless database. You can use the data retrieval API to retrieve objects from the Backendless database. Data in the JSON columns is returned to the client application as either a strongly-typed object or as . Consider the example below:

Suppose the database stores objects in the Person data table. The table declares the profile column of type JSON. The column contains JSON values in the following format:

{
  "age": 55,
  "name": "Bob",
  "address": {
    "city": "Los Angeles",
    "state": "California",
    "street": "123 Santa Monica Blvd."
  },
  "lastname": "Smith",
  "favoriteColors": [
    "Blue", "Red"
  ],
  "favoriteNumbers": [
    13, 21, 88
  ]
}

As you can see the sample JSON value above exhibits the following "qualities":

• literal string and numeric values - age, name and lastname keys.
• an array consisting of strings - favoriteColors
• an array consisting of numbers - favoriteNumbers
• an enclosed JSON object in the address key.

The JSON values and the Person objects may appear as shown below in the database:

Suppose the client application needs to retrieve both Person objects and the corresponding profile values . This can be accomplished with the following code:

const person = await Backendless.Data.of('Person').findById('AE2B0CBD-A5A9-453C-9F9A-E15481DF3C02')
const { profile } = person

console.log('Name:', profile.name)
console.log('Last name:', profile.lastname)
console.log('Age:', profile.age)
console.log('Favorite numbers:', profile.favoriteNumbers)
console.log('Favorite colors:', profile.favoriteColors)

const { address } = profile

console.log('Street:', address.street)
console.log('City:', address.city)
console.log('State:', address.state)

The code produces the following log output:

Name Bob
Last name Smith
Age 55
Favorite numbers [13, 21, 88]
Favorite colors [Blue, Red]
Street 123 Santa Monica Blvd.
City Los Angeles
State California

As you can see, the entire JSON structure is converted to a JavaScript object with all the key/value pairs becoming corresponding entries in the returned object.

Your application may use an alternative approach - representing objects in the Backendless database with strongly-typed classes. In the context of the example above, this means you will have the Person class defined in your application.  See the example below:

Address class:

// declare custom Address class
class Address {
 constructor(address) {
   address = address || {}

   this.street = address.street
   this.city = address.city
   this.state = address.state
 }

 setStreet(street) {
   this.street = street

   return this
 }

 getStreet() {
   return this.street
 }

 setCity(city) {
   this.city = city

   return this
 }

 getCity() {
   return this.city
 }

 setState(state) {
   this.state = state

   return this
 }

 getState() {
   return this.state
 }
}

Profile class - this class will conntain values from JSON

class Profile {
 constructor(profile) {
   profile = profile || {}

   this.name = profile.name
   this.age = profile.age
   this.favoriteNumbers = profile.favoriteNumbers
   this.favoriteColors = profile.favoriteColors
   this.address = new Address(profile.address)
 }

 setName(name) {
   this.name = name

   return this
 }

 getName() {
   return this.name
 }

 setAge(age) {
   this.age = age

   return this
 }

 getAge() {
   return this.age
 }

 setFavoriteNumbers(favoriteNumbers) {
   this.favoriteNumbers = favoriteNumbers

   return this
 }

 getFavoriteNumbers() {
   return this.favoriteNumbers
 }

 setFavoriteColors(favoriteColors) {
   this.favoriteColors = favoriteColors

   return this
 }

 setAddress(address) {
   this.address = new Address(address)

   return this
 }

 getAddress() {
   return this.address
 }
}

Person class - this class is used to represent database objects. Notice the class has the "profile" property, which at the database level is JSON.

class Person {
 constructor(person) {
   person = person || {}

   this.profile = new Profile(person.profile)
 }

 setProfile(profile) {
   this.profile = new Profile(profile)

   return this
 }

 getProfile() {
   return this.profile
 }
}

Code to fetch data from the database and initialize custom class:

const personObject = await Backendless.Data.of('Person').findById('AE2B0CBD-A5A9-453C-9F9A-E15481DF3C02')

const person = new Person(personObject) // initialize person as Person class

In case when the JSON values are represented as strongly-typed classes, Backendless automatically adapts them to the instances of your classes. It is important to maintain the basic fidelity between the JSON types and the corresponding  types as shown below: