Inverted Relation Retrieval¶

Backendless supports a special query syntax for loading a subset of child objects for a specific parent. Consider the following table schemas:

PhoneBook table:

phonebook-table-schema

Contact table:

contact-table-schema

Address table:

address-table-schema

These tables will be used to demonstrate how to query Backendless for conditional related object retrieval.

The Map-based approach does not require you defining classes for the objects stored in Backendless. Instead your code can use Map to to store and retrieve objects in/from Backendless.

Consider the following class definitions for the entities from the diagram: PhoneBook class:

import 'package:backendless_sdk/backendless_sdk.dart';

import 'contact.dart';

@reflector
class PhoneBook {
  String objectId;
  Contact owner;
  List<Contact> contacts;
}

Contact class:

import 'package:backendless_sdk/backendless_sdk.dart';

import 'address.dart';

@reflector
class Contact {
  String objectId;
  String name;
  int age;
  String phone;
  String title;
  Address address;
  DateTime updated;
}

Address class:

import 'package:backendless_sdk/backendless_sdk.dart';

@reflector
class Address {
  String street;
  String city;
  String state;
}

The general structure of a whereClause query to load a collection of child objects for a specific parent object is:

ParentTableName[ relatedColumnName ].parentColumnName COLUMN-VALUE-CONDITION

Both columns relatedColumnName and parentColumnName must be declared in a table with name of ParentTableName. The relatedColumnName must be a relation column. The table relatedColumnName points to is the table where the objects must be loaded from. The examples below demonstrate the usage of this syntax:

Find all contacts in a city for a specific phone book¶

Using Map

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook["objectId"]}' and address.city = 'Smallville'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.of( "Contact" ).find( queryBuilder );

Custom Class

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook.objectId}' and address.city = 'Smallville'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.withClass<Contact>().find( queryBuilder );

Find all contacts for the specific phone book where the city name contains letter 'a'¶

Using Map

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook["objectId"]}' and address.city like '%a%'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.of( "Contact" ).find( queryBuilder );

Custom Class

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook.objectId}' and address.city like '%a%'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.withClass<Contact>().find( queryBuilder );

Find all contacts where age is greater than 20 for a specific phone book¶

Using Map

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook["objectId"]}' and age > 20";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.of( "Contact" ).find( queryBuilder );

Custom Class

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook.objectId}' and age > 20";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.withClass<Contact>().find( queryBuilder );

Find all contacts for a specific phone book where age is within the specified range¶

Using Map

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook["objectId"]}' and age >= 21 and age <= 30";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.of( "Contact" ).find( queryBuilder );

Custom Class

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook.objectId}' and age >= 21 and age <= 30";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.withClass<Contact>().find( queryBuilder );

Find all contacts for a specific phone book where age is greater than 20 and the city is Tokyo¶

Using Map

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook["objectId"]}' and age > 20 and address.city = 'Tokyo'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.of( "Contact" ).find( queryBuilder ); 


String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook.objectId}' and age > 20 and address.city = 'Tokyo'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.withClass<Contact>().find( queryBuilder );

Custom Class

String whereClause = "PhoneBook[contacts].objectId='${savedPhoneBook.objectId}' and age > 20 and address.city = 'Tokyo'";

DataQueryBuilder queryBuilder = DataQueryBuilder()
  ..whereClause = whereClause;
Backendless.data.withClass<Contact>().find( queryBuilder );