Basic Queries

basic queries introduction in most use cases, we require to fetch data from a database with certain conditions these conditions may include complex comparisons and ordering requirements thus, in any application, it is fundamental to construct efficient queries and, at the same time, the database has to be able to execute them as fast as possible the parseswift sdk parseswift sdk does provide the necessary tools for you to construct any query according to the application requirements in this tutorial, we explore these tools and use them in a real world application 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 to create basic queries to retrieve data from 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 constacts app the project template is a contacts app where the user adds a contact’s information to save it on a back4app database on the app’s homescreen you will find a set of buttons for different types of queries using the + + button located on the top right side of the navigation bar, we can add as many contacts contacts as needed quick reference of commands we are going to use for this example, we use the object contact contact 1 import foundation 2 import parseswift 3 4 struct contact parseobject { 5 // required properties from parseobject protocol 6 var originaldata data? 7 var objectid string? 8 var createdat date? 9 var updatedat date? 10 var acl parseacl? 11 12 // custom fields for the contact's information 13 var name string? 14 var birthday date? 15 var numberoffriends int? 16 var favoritefoods \[string]? 17 18 19 } the following methods will allows us to save and query contact contact objects create contact //when creating and saving a new instance of contact we can use 1 var newcontact contact = contact(name "john doe", birthday date(), numberoffriends 5, favoritefoods \["bread", "pizza"]) 2 3 // saves newcontact on your back4app database synchronously and returns the new saved item it throws and error if something went wrong 4 let savedcontact = try? newcontact save() 5 6 // saves newcontact on your back4app database asynchronously, and passes a result\<contact, parseerror> object to the completion block to handle the save process 7 newcontact save { result in 8 // handle the result to check wether the save process was successfull or not 9 } query all //for retrieving all the contact items saved on a back4app database, we construct a query\<contact> object and call the find() method on it 1 let contactsquery = contact query() // a query to fetch all contact items on your back4app database 2 3 // fetches the items synchronously or throws an error if found 4 let fetchedcontacts = 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 } query by name //in order to create a query with a specific condition, we use the static method query( ) provided by the parseobject protocol we pass a queryconstraint object to the method as a parameter this queryconstraint object represents the type of constraint we are imposing on the query for queries involving comparison constraints, the parseswift sdk provides the following methods to create them 1 import parseswift 2 3 // a constraint to retreive all contact items that have exactly the string 'jhon doe' in their 'name' field 4 let constraint1 = try? equalto(key "name", value "john doe") 5 6 // an operator like implementation for the equalto(key\ value ) method 7 let constraint2 queryconstraint = "name" == "jhon doe" 8 9 // a constraint to retrieve all contact items that have the string 'john' in their 'name' field (only workd with string type fields) 10 let constraint3 queryconstraint = containsstring(key "name", substring "jhon") 11 12 let query = contact query(constrint1) // depending on your use case, you can send any of the above constraints as parameter 13 14 // executes the query synchronously it throws an error if something happened 15 let fetchedcontacts = try? query find() 16 17 // executes que query asynchronously and returns a result<\[contact], parseerror> object with the result 18 query find() { result in 19 // handle the result 20 } query by friend count //when we want to query contacts which have a certain amount of friends or more, we do it in the following way 1 import parseswift 2 3 // a constraint to retrieve all contact items that have 30 or more number of friends 4 let constraint1 queryconstraint = "numberoffriends" >= 30 5 6 // a constraint to retrieve all contact items that have more than 30 number of friends 7 let constraint2 queryconstraint = "numberoffriends" > 30 8 9 let query = contact query(constraint1) // depending on your use case, you can send any of the above constraints as parameter 10 11 // executes the query synchronously it throws an error if something happened 12 let fetchedcontacts = try? query find() 13 14 // executes que query asynchronously and returns a result<\[contact], parseerror> object with the result 15 query find() { result in 16 // handle the result 17 } query with ordering //adding an ordering option to queries is straightforward any query\<contact> object has the order( ) method to do so a simple query using the birthday as descending order can be implemented in the folowing way 1 import parseswift 2 3 // a query without order to retrieve all the contact items 4 let unorderedquery = contact query() 5 6 // sorts the result by the brithday field the parameter in the enumeration is the key of the field used to order the results 7 let descendingorder = query\<contact> order descending("birthday") 8 9 let orderedquery = unorderedquery order(\[descendingorder]) // returns a new query with the requested (descending) ordering option 10 11 // executes the query synchronously it throws an error if something happened 12 let orderedcontacts = try? orderedquery find() 13 14 // executes que query asynchronously and returns a result<\[contact], parseerror> object with the result 15 orderedcontacts find() { result in 16 // handle the result 17 } 1 download the contacts 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 to focus on the main objective of this guide, we will only detail the sections strictly related to queries and the parseswift sdk 2 additional crud flow before getting started with queries, it is necessary to have some contacts already saved on your back4app database in the newcontactcontroller newcontactcontroller class, we implement a basic form to add a contact contact to save an instance of a contact contact object, we use the handleaddcontact() handleaddcontact() method implemented in the newcontactcontroller newcontactcontroller class 1 // newcontactcontroller swift file 2 3 4 extension newcontactcontroller { 5 /// retrieves the info the user entered for a new contact and stores it on your back4app database 6 @objc fileprivate func handleaddcontact() { 7 view\ endediting(true) 8 9 // collect the contact's information from the form 10 guard let name = nametextfield text, 11 let numberoffriendsstring = numberoffriendstextfield text, 12 let numberoffriends = int(numberoffriendsstring), 13 let favoritefoods = favoritefoodstextfield text? split(separator ",") else { 14 return showalert(title "error", message "the data you entered is con valid ") 15 } 16 17 // once the contact's information is collected, instantiate a contact object to save it on your back4app database 18 let contact = contact( 19 name name, 20 birthday birthdaydatepicker date, 21 numberoffriends numberoffriends, 22 favoritefoods favoritefoods compactmap { string($0) trimmingcharacters(in whitespaces) } 23 ) 24 25 // save the new contact 26 contact save { \[weak self] result in 27 switch result { 28 case success( ) 29 self? showalert(title "success", message "contact saved ") { 30 self? dismiss(animated true, completion nil) 31 } 32 case failure(let error) 33 self? showalert(title "error", message "failed to save contact \\(error message)") 34 } 35 } 36 } 37 } for more details about this step, you can go to the basic operations guide 3 performing basic queries \ by name the first example we look at is a query that allows us to retrieve contacts which have a specific substring in their name name field in order to do this, we first create a queryconstraint queryconstraint object this object will contain the constraint we want the parseswift sdk parseswift sdk provides the following methods to (indirectly) create a queryconstraint queryconstraint 1 // queryconstraint swift file 2 3 / 4 add a constraint for finding string values that contain a provided substring 5 warning this will be slow for large datasets 6 parameter key the key that the string to match is stored in 7 parameter substring the substring that the value must contain 8 parameter modifiers any of the following supported pcre modifiers (defaults to nil) 9 `i` case insensitive search 10 `m` search across multiple lines of input 11 returns the resulting `queryconstraint` 12 / 13 public func containsstring(key string, substring string, modifiers string? = nil) > queryconstraint 14 15 / 16 add a constraint that requires that a key is equal to a value 17 parameter key the key that the value is stored in 18 parameter value the value to compare 19 returns the same instance of `queryconstraint` as the receiver 20 warning see `equalto` for more information 21 behavior changes based on `parseswift configuration isusingequalqueryconstraint` 22 where isusingequalqueryconstraint == true is known not to work for livequery on 23 parse servers <= 5 0 0 24 / 25 public func == \<t>(key string, value t) > queryconstraint where t encodable for instance, a query that allows us to retrieve all the contact contact ’s with john in their name name field can be created with 1 // create the query sending the constraint as parameter 2 let constraint queryconstraint = containsstring(key "name", substring "john") // the first parameter (key) referres to the name of the field 3 let query = contact query(constrain) 4 5 // retrieve the contacts asynchronously (or sinchronously if needed) 6 query find() { result in 7 // handle the result and do the corresponding ui update 8 } in case the constraint requires the name name field to match exactly a given string, we can use 1 // create the query sending the constraint as parameter 2 let value = "john" 3 let constraint queryconstraint = "name" == value 4 let query = contact query(constrain) \ by number of friends a query with a constraint involving a numerical comparison can be constructed by creating a queryconstraint queryconstraint with 1 / 2 add a constraint that requires that a key is greater than a value 3 parameter key the key that the value is stored in 4 parameter value the value to compare 5 returns the same instance of `queryconstraint` as the receiver 6 / 7 public func > \<t>(key string, value t) > queryconstraint where t encodable 8 9 / 10 add a constraint that requires that a key is greater than or equal to a value 11 parameter key the key that the value is stored in 12 parameter value the value to compare 13 returns the same instance of `queryconstraint` as the receiver 14 / 15 public func >= \<t>(key string, value t) > queryconstraint where t encodable 16 17 / 18 add a constraint that requires that a key is less than a value 19 parameter key the key that the value is stored in 20 parameter value the value to compare 21 returns the same instance of `queryconstraint` as the receiver 22 / 23 public func < \<t>(key string, value t) > queryconstraint where t encodable 24 25 / 26 add a constraint that requires that a key is less than or equal to a value 27 parameter key the key that the value is stored in 28 parameter value the value to compare 29 returns the same instance of `queryconstraint` as the receiver 30 / 31 public func <= \<t>(key string, value t) > queryconstraint where t encodable to query all contacts with 30 or more friends, we use 1 let query = contacts query("numberoffriends" >= 30) 2 3 // retrieve the contacts asynchronously (or sinchronously if needed) 4 query find() { result in 5 // handle the result and do the corresponding ui update 6 } \ ordering query results for ordering the results from a query, the query\<contacts> query\<contacts> object provides the method order( ) order( ) which returns a new query\<contact> query\<contact> object considering the requested ordering option as a parameter, we pass an enumeration ( query\<contact> order query\<contact> order ) to indicate the ordering we want the following snippet applies a descending order based on the birthday birthday field 1 // a query without order to retrieve all the contact items 2 let unorderedquery = contact query() 3 4 // sorts the contacts based on their brithday the parameter in the enumeration is the key of the field used to order the results 5 let descendingorder = query\<contact> order descending("birthday") 6 7 let orderedquery = unorderedquery order(\[descendingorder]) // returns a new query with the requested (descending) ordering option 8 9 // executes que query asynchronously and returns a result<\[contact], parseerror> object with the result 10 orderedcontacts find() { result in 11 // handle the result 12 } in the project example https //github com/templates back4app/ios basic queries example , we implemented the queries mentioned above the contactscontroller contactscontroller class has the method fetchcontacts() fetchcontacts() where you will find the following snippet 1 2 3 class contactscontroller { 4 let querytype querytype 5 6 7 8 private func fetchcontacts() { 9 // we create a query\<contact> according to the querytype enumeration 10 let query query\<contact> = { 11 switch querytype { 12 case byname(let value) 13 return contact query(containsstring(key "name", substring value)) 14 case bynumberoffriends(let quantity) 15 return contact query("numberoffriends" >= quantity) 16 case byordering(let order) 17 let query = contact query() 18 switch order { 19 case ascending return query order(\[ ascending("birthday")]) 20 case descending return query order(\[ descending("birthday")]) 21 } 22 case all 23 return contact query() 24 } 25 }() 26 27 // execute the query 28 query find { \[weak self] result in 29 switch result { 30 case success(let contacts) 31 self? contacts = contacts 32 33 // update the ui 34 dispatchqueue main async { self? tableview\ reloaddata() } 35 case failure(let error) 36 // notify the user about the error that happened during the fetching process 37 self? showalert(title "error", message "failed to retrieve contacts \\(error message)") 38 return 39 } 40 } 41 } 42 } 4 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! using the + + button in the navigation bar, add a counple of contacts and test the different queries