Relationships

relationships introduction relations are a fundamental feature for organizing data objects stored on a database parseswift does provide the necessary tools and methods to establish relations between classes in your back4app database depending on the use case, we can identify the following type of relations 1 1 a relation that only connects two data objects 1\ n a relation between one data object and n n data objects n\ n a relation beween n n data objects to n n data objects as we will see below, implementing 1 1 relations are relatively straightforward for 1\ n and n\ n relations, the implementation involves the parserelation parserelation object provided by parseswift sdk there are additional alternatives for implementing 1\ n and n\ n relations however, due to efficiency and performance, we recommend following our approach this tutorial uses a basic app created in xcode 12 and ios 14 at any time, you can access the complete project via our github repositories ios example repository goal to understand how relations are implemented in a back4app database prerequisites to complete this quickstart, you need xcode an app created at back4app follow the new parse app tutorial to learn how to create a parse app at back4app note follow the install parse sdk (swift) tutorial to create an xcode project connected to back4app understanding our books app the project template is a book app where the user enters a book details to save it on a back4app database on the app’s homescreen you will find the form to do so using the + + button located on the top right side of the navigation bar, we can add as many publishers , genres and authors as needed once the user enters a book, they can use the add book add book button to save the book on their back4app database additionally, the list books list books button will allow us to show all the books the user added and also to see their relationship with the publishers and authors quick reference of commands we are going to use we make use of the objects author author , publisher publisher , isbn isbn and book book genre 1 import foundation 2 import parseswift 3 4 struct genre parseobject { 5 6 7 var name string? 8 9 10 } author 1 import foundation 2 import parseswift 3 4 struct author parseobject { 5 6 7 var name string? 8 9 10 } publisher 1 import foundation 2 import parseswift 3 4 struct publisher parseobject { 5 6 7 var name string? 8 9 10 } isbn 1 import foundation 2 import parseswift 3 4 struct isbn parseobject { 5 6 7 var name string? 8 9 10 } book 1 import foundation 2 import parseswift 3 4 struct book parseobject { 5 6 7 var title string? 8 var publishingyear int? 9 var genre genre? 10 var isbn isbn? 11 12 13 } before storing instances of these objects in a back4app database, all their properties must conform to the codable codable and hashable hashable protocols we make use of the following methods for managing these objects on the back4app database create //when creating a new instance of author we use 1 var newauthor author = author(name "john doe") 2 3 // saves newauthor on your back4app database synchronously and returns the new saved item it throws and error if something went wrong 4 let savedauthor = try? newauthor save() 5 6 // saves newauthor on your back4app database asynchronously, and passes a result\<author, parseerror> object to the completion block to handle the save process 7 newauthor save { result in 8 // handle the result to check wether the save process was successfull or not 9 } read //for reading any of the objects introduced above, we construct the corresponding queries for each of them for author we have 1 let authorquery = author query() // a query to fetch all author items on your back4app database 2 3 // fetches the items synchronously or throws an error if found 4 let fetchedauthors = try? query find() 5 6 // fetches the items asynchronously and calls a completion block passing a result object containing the result of the operation 7 query find { result in 8 // handle the result 9 } add relations //in 1 1 relations, it is sufficient to have the child object as a property of the parent object back4app automatically saves the child object when the parent object is saved for the remaining relations, we use the add( objects ) method via the relation property available in the parent parseobject adding a relation called authors on a book object would look like this 1 let somebook book 2 let authors \[author] 3 4 // adds the relation between somebook and authors under the name 'authors' 5 let booktoauthorsrelation = try? somebook relation? add("authors", objects authors) 6 7 // saves the relation synchronously 8 let updatedsomebook = try? booktoauthorsrelation save() 9 10 // saves the relation asynchronously 11 booktoauthorsrelation save { result in 12 // handle the result 13 } query relations //for 1 1 relations, it is enough to append the include() method in the query for instance, to query all the books together with their isbn relation, we use 1 var query = book query() include("isbn") 2 3 // fetches all books synchronously 4 let books = try? query find() 5 6 // fetches all books asynchronously 7 query find { result in 8 // handle the result 9 } remaining relations //for the remaining relations, we create a query by using the static method queryrelation( ,parent ) provided by the parseobject protocol querying the authors related to a book can be implemented in the following way 1 let somebook book 2 let authors \[author] 3 4 5 // we create a relation (identified by the name 'authors') betwee somebook and a set of authors 6 let booktoauthorsrelation = 7 guard let booktoauthorsrelation = try somebook relation? add("authors", objects authors) // book > author 8 else { 9 fatalerror("failed to add relation") 10 } 11 12 let savedrelation = try booktoauthorsrelation save() // saves the relation synchronously 13 14 booktoauthorsrelation save { result in // saves the relation asynchronously 15 // handle the result 16 } 1 download the books app template the xcode xcode project has the following structure at any time, you can access the complete project via our github repositories ios example repository 2 additional crud flow before going further, it is necessary to implement some crud functions for us to be able to save the author author , publisher publisher and genre genre objects in the maincontroller+parseswift swift maincontroller+parseswift swift file, under an extension for the maincontroller maincontroller class, we implemented the following methods 1 // maincontroller+parseswift swift file 2 extension maincontroller { 3 /// collects the data to save an instance of book on your back4app database 4 func savebook() { 5 view\ endediting(true) 6 7 // 1 first retrieve all the information for the book (title, isbn, etc) 8 guard let booktitle = booktitletextfield text else { 9 return presentalert(title "error", message "invalid book title") 10 } 11 12 guard let isbnvalue = isbntextfield text else { 13 return presentalert(title "error", message "invalid isbn value ") 14 } 15 16 let query = isbn query("value" == isbnvalue) 17 18 guard (try? query first()) == nil else { 19 return presentalert(title "error", message "the entered isbn already exists ") 20 } 21 22 guard let genreobjectid = genreoptionsview\ selectedoptionids first, 23 let genre = genres first(where { $0 objectid == genreobjectid}) 24 else { 25 return presentalert(title "error", message "invalid genre ") 26 } 27 28 guard let publishingyearstring = publishingyeartextfield text, let publishingyear = int(publishingyearstring) else { 29 return presentalert(title "error", message "invalid publishing year ") 30 } 31 32 let authors \[author] = self authoroptionsview\ selectedoptionids compactmap { \[weak self] objectid in 33 self? authors first(where { objectid == $0 objectid }) 34 } 35 36 let publishers \[publisher] = self publisheroptionsview\ selectedoptionids compactmap { \[weak self] objectid in 37 self? publishers first(where { objectid == $0 objectid }) 38 } 39 40 // since we are making multiple requests to back4app, it is better to use synchronous methods and dispatch them on the background queue 41 dispatchqueue global(qos background) async { 42 do { 43 let isbn = isbn(value isbnvalue) // 2 instantiate a new isbn object 44 45 let savedbook = try book( // 3 instantiate a new book object with the corresponding input fields 46 title booktitle, 47 publishingyear publishingyear, 48 genre genre, 49 isbn isbn 50 ) save() // 4 save the new book object 51 52 // here we will implement the relations 53 54 dispatchqueue main async { 55 self presentalert(title "success", message "book saved successfully ") 56 } 57 } catch { 58 dispatchqueue main async { 59 self presentalert(title "error", message "failed to save book \\((error as! parseerror) message)") 60 } 61 } 62 } 63 } 64 65 /// retrieves all the data saved under the genre class in your back4app database 66 func fetchgenres() { 67 let query = genre query() 68 69 query find { \[weak self] result in 70 switch result { 71 case success(let genres) 72 self? genres = genres // when setting self? genres, it triggers the corresponding ui update 73 case failure(let error) 74 self? presentalert(title "error", message error message) 75 } 76 } 77 } 78 79 /// presents a simple alert where the user can enter the name of a genre to save it on your back4app database 80 func handleaddgenre() { 81 // displays a form with a single input and executes the completion block when the user presses the submit button 82 presentform( 83 title "add genre", 84 description "enter a description for the genre", 85 placeholder nil 86 ) { \[weak self] name in 87 guard let name = name else { return } 88 let genre = genre(name name) 89 90 let query = genre query("name" == name) 91 92 guard ((try? query first()) == nil) else { 93 self? presentalert(title "error", message "this genre already exists ") 94 return 95 } 96 97 genre save { \[weak self] result in 98 switch result { 99 case success(let addedgenre) 100 self? presentalert(title "success", message "genre added!") 101 self? genres append(addedgenre) 102 case failure(let error) 103 self? presentalert(title "error", message "failed to save genre \\(error message)") 104 } 105 } 106 } 107 } 108 109 /// retrieves all the data saved under the publisher class in your back4app database 110 func fetchpublishers() { 111 let query = publisher query() 112 113 query find { \[weak self] result in 114 switch result { 115 case success(let publishers) 116 self? publishers = publishers 117 case failure(let error) 118 self? presentalert(title "error", message error message) 119 } 120 } 121 } 122 123 /// presents a simple alert where the user can enter the name of a publisher to save it on your back4app database 124 func handleaddpublisher() { 125 // displays a form with a single input and executes the completion block when the user presses the submit button 126 presentform( 127 title "add publisher", 128 description "enter the name of the publisher", 129 placeholder nil 130 ) { \[weak self] name in 131 guard let name = name else { return } 132 133 let query = publisher query("name" == name) 134 135 guard ((try? query first()) == nil) else { 136 self? presentalert(title "error", message "this publisher already exists ") 137 return 138 } 139 140 let publisher = publisher(name name) 141 142 publisher save { \[weak self] result in 143 switch result { 144 case success(let addedpublisher) 145 self? presentalert(title "success", message "publisher added!") 146 self? publishers append(addedpublisher) 147 case failure(let error) 148 self? presentalert(title "error", message "failed to save publisher \\(error message)") 149 } 150 } 151 } 152 } 153 154 /// retrieves all the data saved under the genre class in your back4app database 155 func fetchauthors() { 156 let query = author query() 157 158 query find { \[weak self] result in 159 switch result { 160 case success(let authors) 161 self? authors = authors 162 case failure(let error) 163 self? presentalert(title "error", message error message) 164 } 165 } 166 } 167 168 /// presents a simple alert where the user can enter the name of an author to save it on your back4app database 169 func handleaddauthor() { 170 // displays a form with a single input and executes the completion block when the user presses the submit button 171 presentform( 172 title "add author", 173 description "enter the name of the author", 174 placeholder nil 175 ) { \[weak self] name in 176 guard let name = name else { return } 177 178 let query = author query("name" == name) 179 180 guard ((try? query first()) == nil) else { 181 self? presentalert(title "error", message "this author already exists ") 182 return 183 } 184 185 let author = author(name name) 186 187 author save { \[weak self] result in 188 switch result { 189 case success(let addedauthor) 190 self? presentalert(title "success", message "author added!") 191 self? authors append(addedauthor) 192 case failure(let error) 193 self? presentalert(title "error", message "failed to save author \\(error message)") 194 } 195 } 196 } 197 } 198 }sw for more details about this step, you can go to the basic operations guide 3 setup the relations before starting to create relations, take a look at the quick reference https //www back4app com/docs/ios/parse swift sdk/data objects/relationships#quick reference section to have an idea about the objects we want to relate to each other in the figure below we show how these objects are related as can be seen, the relations are created by putting the book book object in the middle the arrows show how each object is related to a book book object 4 implementing relations 1 1 case adding 1 1 relations can easlity be achieved by adding a property in the book book object, i e , 1 struct book parseobject { 2 3 4 var isbn isbn? // esablishes a 1 1 relation between book and isbn 5 6 7 } in this case, book book and isbn isbn share a 1 1 relation where book book is identified as the parent and isbn isbn as the child internally, when back4app saves an instance of book book , it saves the isbn isbn object (under the isbn isbn class name) first after this process is complete, back4app continues with the object book book the new book book object is saved in a way that its isbn isbn property is represented by a pointer\<isbn> pointer\<isbn> object a pointer<> pointer<> object allows us to store a unique instance of the isbn isbn object related to its corresponding parent 1\ n case for 1\ n relations, the most efficient way to implement them is via a parserelation\<book> parserelation\<book> object parseswift provides a set of methods to add these types of relationships for any object conforming to the parseobject parseobject protocol for instance, if we want to create a 1\ n 1\ n relation between book book and author author we can use 1 let somebook book 2 let authors \[author] 3 4 5 // we create a relation (identified by the name 'authors') between somebook and a set of authors 6 let booktoauthorsrelation = 7 guard let booktoauthorsrelation = try somebook relation? add("authors", objects authors) // book > author 8 else { 9 fatalerror("failed to add relation") 10 } 11 12 let savedrelation = try booktoauthorsrelation save() // saves the relation synchronously 13 14 booktoauthorsrelation save { result in // saves the relation asynchronously 15 // handle the result 16 } it is straightforward to adapt this snippet for the other relations we have for book book \ putting all together once we established the basic idea to implement relations, we now complete the savebook() savebook() method we enumerate the key points to have in mind during this process 1 extension maincontroller { 2 /// collects the data to save an instance of book on your back4app database 3 func savebook() { 4 5 // 1 first retrieve all the information for the book (booktitle, isbnvalue, etc) 6 7 8 // since we are making multiple requests to back4app, it is better to use synchronous methods and dispatch them on the background queue 9 dispatchqueue global(qos background) async { 10 do { 11 let isbn = isbn(value isbnvalue) // 2 instantiate a new isbn object 12 13 let savedbook = try book( // 3 instantiate a new book object with the corresponding input fields 14 title booktitle, 15 publishingyear publishingyear, 16 genre genre, 17 isbn isbn 18 ) save() // 4 save the new book object 19 20 // 5 add the corresponding relations for new book object 21 guard let booktoauthorsrelation = try savedbook relation? add("authors", objects authors), // book > author 22 let bootktopublishersrelation = try savedbook relation? add("publishers", objects publishers), // book > publisher 23 let genrerelation = try genre relation? add("books", objects \[savedbook]) // genre > book 24 else { 25 return dispatchqueue main async { 26 self presentalert(title "error", message "failed to add relations") 27 } 28 } 29 30 // 6 save the relations 31 = try booktoauthorsrelation save() 32 = try bootktopublishersrelation save() 33 = try genrerelation save() 34 35 dispatchqueue main async { 36 self presentalert(title "success", message "book saved successfully ") 37 } 38 } catch { 39 dispatchqueue main async { 40 self presentalert(title "error", message "failed to save book \\((error as! parseerror) message)") 41 } 42 } 43 } 44 } 45 46 47 } 5 querying relations for 1 1 relations, given the parent book book and its child isbn isbn , we query the corresponding child by including it in the book book query 1 let query = book query() include("isbn") 2 3 let books = try query find() // retrieves synchronously all the books together with its isbn 4 5 query find { result in // retrieves asynchronously all the books together with its isbn 6 // handle the result 7 } 8 with this, all the books from the query will also have the isbn property set properly with the related isbn object on the other hand, in order to retrieve objects with a 1\ n relation, the parseobject protocol provides the static method queryrelation( ,parent ) by providing the name of the relation (as the first parameter) and the parent, this method allows us to create the required query for instance, to retrieve all the author ’s related to a specific book we can use the following snippet 1 let book book // book from which we are trying to retrieve its related authors 2 3 do { 4 let authorsquery = try author queryrelations("authors", parent book) // 'authors' is the name of the relation it was saved with 5 6 authorsquery find { \[weak self] result in 7 switch result { 8 case success(let authors) 9 self? authors = authors 10 11 dispatchqueue main async { 12 // update the ui 13 } 14 case failure(let error) 15 dispatchqueue main async { 16 self? presentalert(title "error", message "failed to retrieve authors \\(error message)") 17 } 18 } 19 } 20 } 21 } catch { 22 if let error = error as? parseerror { 23 presentalert(title "error", message "failed to retrieve authors \\(error message)") 24 } else { 25 presentalert(title "error", message "failed to retrieve authors \\(error localizeddescription)") 26 } 27 } similarly, we can query other related objects such as publisher in the bookdetailscontroller swift file we implement these queries to display the relation a book has with authors and publishers 1 // bookdetailscontroller swift file 2 class bookdetailscontroller uitableviewcontroller { 3 4 5 /// retrieves the book's details, i e , its relation with authors and publishers 6 private func fetchdetails() { 7 do { 8 // constructs the relations you want to query 9 let publishersquery = try publisher queryrelations("publishers", parent book) 10 let authorsquery = try author queryrelations("authors", parent book) 11 12 // obtains the publishers related to book and display them on the tableview, it presents an error if happened 13 publishersquery find { \[weak self] result in 14 switch result { 15 case success(let publishers) 16 self? publishers = publishers 17 18 // update the ui 19 dispatchqueue main async { 20 self? tableview\ reloadsections(indexset(\[section publisher rawvalue]), with none) 21 } 22 case failure(let error) 23 dispatchqueue main async { 24 self? presentalert(title "error", message "failed to retrieve publishers \\(error message)") 25 } 26 } 27 } 28 29 // obtains the authors related to book and display them on the tableview, it presents an error if happened 30 authorsquery find { \[weak self] result in 31 switch result { 32 case success(let authors) 33 self? authors = authors 34 35 // update the ui 36 dispatchqueue main async { 37 self? tableview\ reloadsections(indexset(\[section author rawvalue]), with none) 38 } 39 case failure(let error) 40 dispatchqueue main async { 41 self? presentalert(title "error", message "failed to retrieve authors \\(error message)") 42 } 43 } 44 } 45 } catch { // if there was an error during the creation of the queries, this block should catch it 46 if let error = error as? parseerror { 47 presentalert(title "error", message "failed to retrieve authors \\(error message)") 48 } else { 49 presentalert(title "error", message "failed to retrieve authors \\(error localizeddescription)") 50 } 51 } 52 } 53 54 55 } 6 run the app! before pressing the run button on xcode xcode , do not forget to configure your back4app back4app application in the appdelegate appdelegate class! you have to add a couple of genre ’s, publisher ’s and author ’s before adding a new book then, you can start entering a book’s information to save it on your back4app database once you have saved one book, open your back4app dashboard https //parse dashboard back4app com/apps and go to your application linked to the xcode project in the database section, you will find the class book book where all the books created by the ios ios app are stored additionally, you can see that back4app automatically created the class isbn isbn in order to relate it with its corresponding book book object if you go back to the book book class, you can identify the data types for each type of relation in the case of isbn isbn and genre genre , the data type is a pointer<> pointer<> on the other hand, for relations like author author and publisher publisher , the data type is relation<> relation<> this is a key difference to have in mind when constructing relations