Relational Queries
Relational Query in React Native using Parse
Introduction
In this guide, you will perform relational queries in Parse and implement a React Native component using these queries. You will learn how to set up and query realistic data using Back4App and React Native.
Prerequisites
Goal
Query relational data stored on Back4App from a React Native App.
1 - Understanding the Parse.Query class
Any Parse query operation uses the Parse.Query object type, which will help you retrieve specific data from your database throughout your app. It is crucial to know that a Parse.Query will only resolve after calling a retrieve method (like Parse.Query.find or Parse.Query.first), so a query can be set up and several modifiers can be chained before actually being called.
To create a new Parse.Query, you need to pass as a parameter the desired Parse.Object subclass, which is the one that will contain your query results. An example query can be seen below, in which a fictional Book subclass is being queried.
1 // This will create your query
2 let parseQuery = new Parse.Query("Book");
3 // The query will resolve only after calling this method
4 let queryResult = await parseQuery.find();You can read more about the Parse.Query class here at the official documentation.
2 - Save some data on Back4App
Let’s create an assortment of classes, which will be the target of our queries in this guide. The classes are: Author, Book, Publisher and BookStore, in which Book has a 1:N relation with Publisher and N:N with Author, and BookStore has an N:N relation with Book.
On Parse JS Console is possible to run JavaScript code directly, querying and updating your application database contents using the JS SDK commands. Run the code below from your JS Console and insert the data on Back4App. Here is how the JS console looks like in your dashboard:
Go ahead and create the classes with the following example content:
1 // Add objects and create tables
2 // Authors
3 const AuthorA = new Parse.Object('Author');
4 AuthorA.set('name', 'Aaron Writer');
5 await AuthorA.save();
6
7 const AuthorB = new Parse.Object('Author');
8 AuthorB.set('name', 'Beatrice Novelist');
9 await AuthorB.save();
10
11 const AuthorC = new Parse.Object('Author');
12 AuthorC.set('name', 'Casey Columnist');
13 await AuthorC.save();
14
15 // Publishers
16 const PublisherA = new Parse.Object('Publisher');
17 PublisherA.set('name', 'Acacia Publishings');
18 await PublisherA.save();
19
20 const PublisherB = new Parse.Object('Publisher');
21 PublisherB.set('name', 'Birch Distributions');
22 await PublisherB.save();
23
24 // Books
25 const BookA = new Parse.Object('Book');
26 BookA.set('title', 'A Love Story');
27 BookA.set('publisher', PublisherA);
28 BookA.set('publishingDate', new Date('05/07/1998'));
29 const BookARelation = BookA.relation("authors");
30 BookARelation.add(AuthorA);
31 await BookA.save();
32
33 const BookB = new Parse.Object('Book');
34 BookB.set('title', 'Benevolent Elves');
35 BookB.set('publisher', PublisherB);
36 BookB.set('publishingDate', new Date('11/31/2008'));
37 const BookBRelation = BookB.relation("authors");
38 BookBRelation.add(AuthorB);
39 await BookB.save();
40
41 const BookC = new Parse.Object('Book');
42 BookC.set('title', 'Can You Believe It?');
43 BookC.set('publisher', PublisherB);
44 BookC.set('publishingDate', new Date('08/21/2018'));
45 const BookCRelation = BookC.relation("authors");
46 BookCRelation.add(AuthorA);
47 BookCRelation.add(AuthorC);
48 await BookC.save();
49
50 // BookStore
51 const BookStoreA = new Parse.Object('BookStore');
52 BookStoreA.set('name', 'Books of Love');
53 const BookStoreARelation = BookStoreA.relation("books");
54 BookStoreARelation.add(BookA);
55 await BookStoreA.save();
56
57 const BookStoreB = new Parse.Object('BookStore');
58 BookStoreB.set('name', 'Fantasy Books');
59 const BookStoreBRelation = BookStoreB.relation("books");
60 BookStoreBRelation.add(BookB);
61 await BookStoreB.save();
62
63 const BookStoreC = new Parse.Object('BookStore');
64 BookStoreC.set('name', 'General Books');
65 const BookStoreCRelation = BookStoreC.relation("books");
66 BookStoreCRelation.add(BookA);
67 BookStoreCRelation.add(BookC);
68 await BookStoreC.save();
69
70 console.log('Success');3 - Query the data
Now that you have populated all the classes, we can now perform some relational queries in it. Let’s begin by filtering Book results by the publisher, searching for the ones that belong to the Publisher “Acacia Publishings” (or “Publisher A”) using the basic Parse.Query.equalTo method:
1 // Get PublisherA object
2 const PublisherAQuery = new Parse.Query('Publisher');
3 PublisherAQuery.equalTo('name', 'Acacia Publishings');
4 const PublisherA = await PublisherAQuery.first();
5
6 // Query Books with PublisherA
7 const bookQuery = new Parse.Query('Book');
8 bookQuery.equalTo('publisher', PublisherA);
9 let queryResults = await bookQuery.find();
10
11 // Let's show the results
12 for (let result of queryResults) {
13 // You access `Parse.Objects` attributes by using `.get`
14 console.log(result.get('title'));
15 };Let’s now query which BookStore objects contain Book objects with publishing date greater than 01/01/2010, using an inner query with the Parse.Query.greaterThan method and then the Parse.Query.matchesQuery method:
1 // Create inner Book query
2 const bookQuery = new Parse.Query('Book');
3 bookQuery.greaterThan('publishingDate', new Date('01/01/2010'));
4
5 // Query BookStore using inner Book query
6 const bookStoreQuery = new Parse.Query('BookStore');
7 bookStoreQuery.matchesQuery('books', bookQuery);
8 let queryResults = await bookStoreQuery.find();
9
10 // Let's show the results
11 for (let result of queryResults) {
12 // You access `Parse.Objects` attributes by using `.get`
13 console.log(result.get('name'));
14 };Let’s now create a more complex relational query, looking for BookStore objects that have at least one Book written by Author “Aaron Writer” (or “AuthorA”), using equalTo and matchesQuery:
1 // Get AuthorA object
2 const AuthorAQuery = new Parse.Query('Author');
3 AuthorAQuery.equalTo('name', 'Aaron Writer');
4 const AuthorA = await AuthorAQuery.first();
5
6 // Create inner Book query
7 const bookQuery = new Parse.Query('Book');
8 bookQuery.equalTo('authors', AuthorA);
9
10 // Query BookStore using inner Book query
11 const bookStoreQuery = new Parse.Query('BookStore');
12 bookStoreQuery.matchesQuery('books', bookQuery);
13 let queryResults = await bookStoreQuery.find();
14
15 // Let's show the results
16 for (let result of queryResults) {
17 // You access `Parse.Objects` attributes by using `.get`
18 console.log(result.get('name'));
19 };4 - Query from a React Native component
Let’s now use our example queries inside a component in React Native, with a simple interface having a list showing results and also buttons for calling the queries. This is how the component code is laid out, note the doQuery functions, containing the example code from before.
1 import React, {FC, ReactElement, useState} from 'react';
2 import {Alert, Image, View, ScrollView, StyleSheet} from 'react-native';
3 import Parse from 'parse/react-native';
4 import {
5 List,
6 Title,
7 Button as PaperButton,
8 Text as PaperText,
9 } from 'react-native-paper';
10
11 export const QueryList: FC<{}> = ({}): ReactElement => {
12 // State variable
13 const [queryResults, setQueryResults] = useState(null);
14
15 const doQueryA = async function (): Promise<boolean> {
16 // Get PublisherA object
17 const PublisherAQuery: Parse.Query = new Parse.Query('Publisher');
18 PublisherAQuery.equalTo('name', 'Acacia Publishings');
19 const PublisherA: Parse.Object = await PublisherAQuery.first();
20
21 // Query Books with PublisherA
22 const bookQuery: Parse.Query = new Parse.Query('Book');
23 bookQuery.equalTo('publisher', PublisherA);
24
25 try {
26 let results: [Parse.Object] = await bookQuery.find();
27 setQueryResults(results);
28 return true;
29 } catch (error) {
30 // Error can be caused by lack of Internet connection
31 Alert.alert('Error!', error.message);
32 return false;
33 }
34 };
35
36 const doQueryB = async function (): Promise<boolean> {
37 // Create inner Book query
38 const bookQuery: Parse.Query = new Parse.Query('Book');
39 bookQuery.greaterThan('publishingDate', new Date('01/01/2010'));
40
41 // Query BookStore using inner Book query
42 const bookStoreQuery: Parse.Query = new Parse.Query('BookStore');
43 bookStoreQuery.matchesQuery('books', bookQuery);
44
45 try {
46 let results: [Parse.Object] = await bookStoreQuery.find();
47 setQueryResults(results);
48 return true;
49 } catch (error) {
50 // Error can be caused by lack of Internet connection
51 Alert.alert('Error!', error.message);
52 return false;
53 }
54 };
55
56 const doQueryC = async function (): Promise<boolean> {
57 // Get AuthorA object
58 const AuthorAQuery: Parse.Query = new Parse.Query('Author');
59 AuthorAQuery.equalTo('name', 'Aaron Writer');
60 const AuthorA: Parse.Object = await AuthorAQuery.first();
61
62 // Create inner Book query
63 const bookQuery: Parse.Query = new Parse.Query('Book');
64 bookQuery.equalTo('authors', AuthorA);
65
66 // Query BookStore using inner Book query
67 const bookStoreQuery: Parse.Query = new Parse.Query('BookStore');
68 bookStoreQuery.matchesQuery('books', bookQuery);
69
70 try {
71 let results: [Parse.Object] = await bookStoreQuery.find();
72 setQueryResults(results);
73 return true;
74 } catch (error) {
75 // Error can be caused by lack of Internet connection
76 Alert.alert('Error!', error.message);
77 return false;
78 }
79 };
80
81 const clearQueryResults = async function (): Promise<boolean> {
82 setQueryResults(null);
83 return true;
84 };
85
86 return (
87 <>
88 <View style={Styles.header}>
89 <Image
90 style={Styles.header_logo}
91 source={ {
92 uri:
93 'https://blog.back4app.com/wp-content/uploads/2019/05/back4app-white-logo-500px.png',
94 } }
95 />
96 <PaperText style={Styles.header_text}>
97 <PaperText style={Styles.header_text_bold}>
98 {'React Native on Back4App - '}
99 </PaperText>
100 {' Relational Queries'}
101 </PaperText>
102 </View>
103 <ScrollView style={Styles.wrapper}>
104 <View>
105 <Title>{'Result List'}</Title>
106 {/* Query list */}
107 {queryResults !== null &&
108 queryResults !== undefined &&
109 queryResults.map((result: Parse.Object) => (
110 <List.Item
111 key={result.id}
112 title={
113 result.get('name') !== undefined
114 ? result.get('name')
115 : result.get('title')
116 }
117 titleStyle={Styles.list_text}
118 style={Styles.list_item}
119 />
120 ))}
121 {queryResults === null ||
122 queryResults === undefined ||
123 (queryResults !== null &&
124 queryResults !== undefined &&
125 queryResults.length <= 0) ? (
126 <PaperText>{'No results here!'}</PaperText>
127 ) : null}
128 </View>
129 <View>
130 <Title>{'Query buttons'}</Title>
131 <PaperButton
132 onPress={() => doQueryA()}
133 mode="contained"
134 icon="search-web"
135 color={'#208AEC'}
136 style={Styles.list_button}>
137 {'Query A'}
138 </PaperButton>
139 <PaperButton
140 onPress={() => doQueryB()}
141 mode="contained"
142 icon="search-web"
143 color={'#208AEC'}
144 style={Styles.list_button}>
145 {'Query B'}
146 </PaperButton>
147 <PaperButton
148 onPress={() => doQueryC()}
149 mode="contained"
150 icon="search-web"
151 color={'#208AEC'}
152 style={Styles.list_button}>
153 {'Query C'}
154 </PaperButton>
155 <PaperButton
156 onPress={() => clearQueryResults()}
157 mode="contained"
158 icon="delete"
159 color={'#208AEC'}
160 style={Styles.list_button}>
161 {'Clear Results'}
162 </PaperButton>
163 </View>
164 </ScrollView>
165 </>
166 );
167 };
168
169 // These define the screen component styles
170 const Styles = StyleSheet.create({
171 header: {
172 alignItems: 'center',
173 paddingTop: 30,
174 paddingBottom: 50,
175 backgroundColor: '#208AEC',
176 },
177 header_logo: {
178 height: 50,
179 width: 220,
180 resizeMode: 'contain',
181 },
182 header_text: {
183 marginTop: 15,
184 color: '#f0f0f0',
185 fontSize: 16,
186 },
187 header_text_bold: {
188 color: '#fff',
189 fontWeight: 'bold',
190 },
191 wrapper: {
192 width: '90%',
193 alignSelf: 'center',
194 },
195 list_button: {
196 marginTop: 6,
197 marginLeft: 15,
198 height: 40,
199 },
200 list_item: {
201 borderBottomWidth: 1,
202 borderBottomColor: 'rgba(0, 0, 0, 0.12)',
203 },
204 list_text: {
205 fontSize: 15,
206 },
207 });This is how the component should look like after rendering and querying by one of the query functions:
Conclusion
At the end of this guide, you learned how relational queries work on Parse and how to perform them on Back4App from a React Native App. In the next guide, you will learn how to work with Users in Parse.