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 to complete this tutorial, you will need a react native app created and connected to back4app if you want to test/use the screen layout provided by this guide, you should set up the react native paper react native paper library 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 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 parse query will only resolve after calling a retrieve method (like parse query find parse query find or parse query first 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 parse query , you need to pass as a parameter the desired parse object 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 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 parse query class here at the official documentation https //parseplatform org/parse sdk js/api/master/parse query html 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 author , book book , publisher publisher and bookstore bookstore , in which book book has a 1\ n relation with publisher publisher and n\ n with author author , and bookstore bookstore has an n\ n relation with book 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&#x9; 7 const authorb = new parse object('author'); 8 authorb set('name', 'beatrice novelist'); 9 await authorb save(); 10&#x9; 11 const authorc = new parse object('author'); 12 authorc set('name', 'casey columnist'); 13 await authorc save(); 14&#x9; 15 // publishers 16 const publishera = new parse object('publisher'); 17 publishera set('name', 'acacia publishings'); 18 await publishera save(); 19&#x9; 20 const publisherb = new parse object('publisher'); 21 publisherb set('name', 'birch distributions'); 22 await publisherb save(); 23&#x9; 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&#x9; 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&#x9; 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&#x9; 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&#x9; 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&#x9; 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&#x9; 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 book results by the publisher, searching for the ones that belong to the publisher publisher “acacia publishings” (or “publisher a”) using the basic parse query equalto 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&#x9; 6 // query books with publishera 7 const bookquery = new parse query('book'); 8 bookquery equalto('publisher', publishera); 9 let queryresults = await bookquery find(); 10&#x9; 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 bookstore objects contain book book objects with publishing date greater than 01/01/2010, using an inner query with the parse query greaterthan parse query greaterthan method and then the parse query matchesquery 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&#x9; 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&#x9; 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 bookstore objects that have at least one book book written by author author “aaron writer” (or “authora”), using equalto equalto and matchesquery 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&#x9; 6 // create inner book query 7 const bookquery = new parse query('book'); 8 bookquery equalto('authors', authora); 9&#x9; 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&#x9; 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 doquery functions, containing the example code from before javascript 1 import react, {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&#x9; 11 export const querylist = () => { 12 // state variable 13 const \[queryresults, setqueryresults] = usestate(null); 14&#x9; 15 const doquerya = async function () { 16 // get publishera object 17 const publisheraquery = new parse query('publisher'); 18 publisheraquery equalto('name', 'acacia publishings'); 19 const publishera = await publisheraquery first(); 20&#x9; 21 // query books with publishera 22 const bookquery = new parse query('book'); 23 bookquery equalto('publisher', publishera); 24&#x9; 25 try { 26 let results = 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&#x9; 36 const doqueryb = async function () { 37 // create inner book query 38 const bookquery = new parse query('book'); 39 bookquery greaterthan('publishingdate', new date('01/01/2010')); 40&#x9; 41 // query bookstore using inner book query 42 const bookstorequery = new parse query('bookstore'); 43 bookstorequery matchesquery('books', bookquery); 44&#x9; 45 try { 46 let results = 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&#x9; 56 const doqueryc = async function () { 57 // get authora object 58 const authoraquery = new parse query('author'); 59 authoraquery equalto('name', 'aaron writer'); 60 const authora = await authoraquery first(); 61&#x9; 62 // create inner book query 63 const bookquery = new parse query('book'); 64 bookquery equalto('authors', authora); 65&#x9; 66 // query bookstore using inner book query 67 const bookstorequery = new parse query('bookstore'); 68 bookstorequery matchesquery('books', bookquery); 69&#x9; 70 try { 71 let results = 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&#x9; 81 const clearqueryresults = async function () { 82 setqueryresults(null); 83 return true; 84 }; 85&#x9; 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) => ( 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&#x9; 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 });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&#x9; 11 export const querylist fc<{}> = ({}) reactelement => { 12 // state variable 13 const \[queryresults, setqueryresults] = usestate(null); 14&#x9; 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&#x9; 21 // query books with publishera 22 const bookquery parse query = new parse query('book'); 23 bookquery equalto('publisher', publishera); 24&#x9; 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&#x9; 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&#x9; 41 // query bookstore using inner book query 42 const bookstorequery parse query = new parse query('bookstore'); 43 bookstorequery matchesquery('books', bookquery); 44&#x9; 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&#x9; 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&#x9; 62 // create inner book query 63 const bookquery parse query = new parse query('book'); 64 bookquery equalto('authors', authora); 65&#x9; 66 // query bookstore using inner book query 67 const bookstorequery parse query = new parse query('bookstore'); 68 bookstorequery matchesquery('books', bookquery); 69&#x9; 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&#x9; 81 const clearqueryresults = async function () promise\<boolean> { 82 setqueryresults(null); 83 return true; 84 }; 85&#x9; 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&#x9; 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