React Native中Parse SDK的一对一关系的实现和查询

9min
一对一关系
﻿
介绍
在许多后端的核心，您会发现存储数据的能力。使用Parse，您可以存储数据对象并建立它们之间的关系。数据关系标准化了每个数据对象与其他对象的关系或关联。这在构建和运行复杂查询时可以为您提供额外的能力。主要有三种关系类型：
﻿一对多, 一个对象可以与多个其他对象相关联；
﻿一对一, 在两个对象之间建立直接关系，仅限于这两个对象；
﻿多对多, 可以在多个对象之间创建许多复杂的关系。
在本指南中，我们将重点关注一对一关系。这些关系在涉及敏感数据和用户管理的应用程序中很常见，这些应用程序需要强制执行唯一字段，例如身份证号码和电话号码。数据存储后端通常会要求明确声明这些关联，而Parse没有自动实现这些关联的解决方案。
然而，可以通过在服务器上使用 Parse.Cloud 函数来实现Parse中的一对一关系，确保在保存数据之前表关系保持唯一。这是通过在两个相关类中创建 beforeSave 函数来完成的，如果父类在子类中已经拥有一个实例，则阻止保存，反之亦然。
您还可以在应用程序的Parse代码中处理这些情况，在保存之前查询服务器并确保该关系。这将是本指南中展示的方法，但请注意，使用云函数要干净得多，并且更为推荐。
在本指南中，您将实现一个包含三种主要数据关联的React Native图书注册应用程序。您将学习如何使用Back4App和React Native创建和查询一对一数据关系。
您可以随时通过我们的GitHub仓库访问此项目，以查看样式和完整代码。
﻿JavaScript示例仓库﻿
﻿TypeScript示例仓库﻿
前提条件
要完成本教程，您需要：
一个创建并连接到 Back4App 的 React Native 应用.
如果您想测试/使用本指南提供的屏幕布局，您应该设置react-native-paper﻿库.
目标
在一个现实场景中，使用 Parse 在 React Native 中执行和演示一对一数据库关系。
- 理解 Book 类
由于在本指南中我们将使用一本书的注册应用示例，您需要首先了解该数据库中对象关系的布局。您将使用的主要对象类是 书籍 类，它将存储注册中的每本书条目。这是其他四个对象类：
﻿出版商: 书籍出版商名称，与书籍﻿
﻿类型: 书籍类型，与书籍. 请注意，在本示例中我们将考虑一本书只能有一个类型；
﻿作者: 书籍作者，与书籍, 因为一本书可以有多个作者，而一个作者也可以有多本书；
﻿ISDB: 书籍的ISDB识别号码，与书籍, 因为这个号码对于每本书都是唯一的。
以下是这些数据库表的可视化表示：
﻿
为了简单起见，我们将假设每个对象类只有一个字符串类型的 名称 属性 (标题 用于 书籍），除了任何额外的关系属性。
- 创建一对多关系
在进入这一步之前，我们建议您克隆并运行 React Native 库应用示例 (JavaScript 示例库, TypeScript 示例库)。该应用程序有两个主要屏幕：一个负责列出注册的书籍，另一个负责创建新书。在书籍注册表单中，有直接链接到其他相关对象，以及一个 文本输入 字段分配给书籍的 ISBD 值，该值将用于创建您的一对一关系。
﻿
让我们看看在提交此表单时调用的书籍创建方法：
JavaScript
TodoList.tsx
const createBook = async function () {
 try {
   // These values come from state variables linked to
   // the screen form fields, retrieving the user choices
   // as a complete Parse.Object, when applicable;
   const bookTitleValue = bookTitle;
   const bookISBDValue = bookISBD;
   // For example, bookPublisher holds the value from
   // RadioButton.Group field with its options being every
   // Publisher parse object instance saved on server, which is
   // queried on screen load via useEffect
   const bookPublisherObject = bookPublisher;
   const bookGenreObject = bookGenre;
   // bookAuthors can be an array of Parse.Objects, since the book
   // may have more than one Author
   const bookAuthorsObjects = bookAuthors;

   // Creates a new parse object instance
   let Book = new Parse.Object('Book');

   // Set data to parse object
   // Simple title field
   Book.set('title', bookTitleValue);

   // 1:1 relation, need to check for uniqueness of value before creating a new ISBD object
   let isbdQuery = new Parse.Query('ISBD');
   isbdQuery.equalTo('name', bookISBDValue);
   let isbdQueryResult = await isbdQuery.first();
   if (isbdQueryResult !== null && isbdQueryResult !== undefined) {
     // If first returns a valid object instance, it means that there
     // is at least one instance of ISBD with the informed value
     Alert.alert(
       'Error!',
       'There is already an ISBD instance with this value!',
     );
     return false;
   } else {
     // Create a new ISBD object instance to create a one-to-one relation on saving
     let ISBD = new Parse.Object('ISBD');
     ISBD.set('name', bookISBDValue);
     ISBD = await ISBD.save();
     // Set the new object to the new book object ISBD field
     Book.set('isbd', ISBD);
   }

   // One-to-many relations can be set in two ways:
   // add direct object to field (Parse will convert to pointer on save)
   Book.set('publisher', bookPublisherObject);
   // or add pointer to field
   Book.set('genre', bookGenreObject.toPointer());

   // many-to-many relation
   // Create a new relation so data can be added
   let authorsRelation = Book.relation('authors');
   // bookAuthorsObjects is an array of Parse.Objects,
   // you can add to relation by adding the whole array or object by object
   authorsRelation.add(bookAuthorsObjects);

   // After setting the values, save it on the server
   try {
     await Book.save();
     // Success
     Alert.alert('Success!');
     navigation.goBack();
     return true;
   } catch (error) {
     // Error can be caused by lack of Internet connection
     Alert.alert('Error!', error.message);
     return false;
   }
 } catch (error) {
   // Error can be caused by lack of value selection
   Alert.alert(
     'Error!',
     'Make sure to select valid choices in Publisher, Genre and Author fields!',
   );
   return false;
 }
};
const createBook = async function () {
 try {
   // These values come from state variables linked to
   // the screen form fields, retrieving the user choices
   // as a complete Parse.Object, when applicable;
   const bookTitleValue = bookTitle;
   const bookISBDValue = bookISBD;
   // For example, bookPublisher holds the value from
   // RadioButton.Group field with its options being every
   // Publisher parse object instance saved on server, which is
   // queried on screen load via useEffect
   const bookPublisherObject = bookPublisher;
   const bookGenreObject = bookGenre;
   // bookAuthors can be an array of Parse.Objects, since the book
   // may have more than one Author
   const bookAuthorsObjects = bookAuthors;

   // Creates a new parse object instance
   let Book = new Parse.Object('Book');

   // Set data to parse object
   // Simple title field
   Book.set('title', bookTitleValue);

   // 1:1 relation, need to check for uniqueness of value before creating a new ISBD object
   let isbdQuery = new Parse.Query('ISBD');
   isbdQuery.equalTo('name', bookISBDValue);
   let isbdQueryResult = await isbdQuery.first();
   if (isbdQueryResult !== null && isbdQueryResult !== undefined) {
     // If first returns a valid object instance, it means that there
     // is at least one instance of ISBD with the informed value
     Alert.alert(
       'Error!',
       'There is already an ISBD instance with this value!',
     );
     return false;
   } else {
     // Create a new ISBD object instance to create a one-to-one relation on saving
     let ISBD = new Parse.Object('ISBD');
     ISBD.set('name', bookISBDValue);
     ISBD = await ISBD.save();
     // Set the new object to the new book object ISBD field
     Book.set('isbd', ISBD);
   }

   // One-to-many relations can be set in two ways:
   // add direct object to field (Parse will convert to pointer on save)
   Book.set('publisher', bookPublisherObject);
   // or add pointer to field
   Book.set('genre', bookGenreObject.toPointer());

   // many-to-many relation
   // Create a new relation so data can be added
   let authorsRelation = Book.relation('authors');
   // bookAuthorsObjects is an array of Parse.Objects,
   // you can add to relation by adding the whole array or object by object
   authorsRelation.add(bookAuthorsObjects);

   // After setting the values, save it on the server
   try {
     await Book.save();
     // Success
     Alert.alert('Success!');
     navigation.goBack();
     return true;
   } catch (error) {
     // Error can be caused by lack of Internet connection
     Alert.alert('Error!', error.message);
     return false;
   }
 } catch (error) {
   // Error can be caused by lack of value selection
   Alert.alert(
     'Error!',
     'Make sure to select valid choices in Publisher, Genre and Author fields!',
   );
   return false;
 }
};
﻿
看看 bookISBDValue 是如何设置为新的书籍 Parse.Object 实例的。在 Parse 中创建和保存一对一和一对多关系是类似的过程，在这个过程中，你将 Parse.Object 实例作为参数传递，使用 Parse.Object.set 方法，该方法接受两个参数：字段名称和要设置的值。
这里的关键是，在保存之前，您需要确保您的数据库中没有包含所提供的 ISBD ID 字符串值的对象，并且没有与之相关的 书籍 对象。第二部分在这种情况下总是成立，因为您每次都在创建一个新的 书籍 对象。通过使用以下高亮的查询，可以强制 ISBD 的唯一性:
JavaScript
TodoList.tsx
let isbdQuery = new Parse.Query('ISBD');
isbdQuery.equalTo('name', bookISBDValue);
let isbdQueryResult = await isbdQuery.first();
if (isbdQueryResult !== null && isbdQueryResult !== undefined) {
 // If first returns a valid object instance, it means that there
 // is at least one instance of ISBD with the informed value
 Alert.alert(
   'Error!',
   'There is already an ISBD instance with this value!',
 );
 return false;
}
let isbdQuery = new Parse.Query('ISBD');
isbdQuery.equalTo('name', bookISBDValue);
let isbdQueryResult = await isbdQuery.first();
if (isbdQueryResult !== null && isbdQueryResult !== undefined) {
 // If first returns a valid object instance, it means that there
 // is at least one instance of ISBD with the informed value
 Alert.alert(
   'Error!',
   'There is already an ISBD instance with this value!',
 );
 return false;
}
﻿
成功保存对象后，Parse 将创建一个指针数据类型列，并在您的仪表板上创建一个直接链接，以便快速访问。
﻿
- 查询一对一关系
查询一对一相关对象非常简单，因为大部分由 Parse 处理。看看书籍注册列表屏幕中的查询功能：
JavaScript
TodoList.tsx
const queryBooks = async function (): Promise<boolean> {
 // These values come from state variables linked to
 // the screen query RadioButton.Group fields, with its options being every
 // parse object instance saved on server from the referred class, which is
 // queried on screen load via useEffect; these variables retrievie the user choices
 // as a complete Parse.Object;
 const queryPublisherValue: Parse.Object = queryPublisher;
 const queryGenreValue: Parse.Object = queryGenre;
 const queryAuthorValue: Parse.Object = queryAuthor;
 const queryIsbdValue: Parse.Object = queryIsbd;

 // Reading parse objects is done by using Parse.Query
 const parseQuery: Parse.Query = new Parse.Query('Book');

 // One-to-many queries
 if (queryPublisherValue !== '') {
   parseQuery.equalTo('publisher', queryPublisherValue);
 }
 if (queryGenreValue !== '') {
   parseQuery.equalTo('genre', queryGenreValue);
 }

 // One-to-one query
 if (queryIsbdValue !== '') {
   parseQuery.equalTo('isbd', queryIsbdValue);
 }

 // Many-to-many query
 // In this case, we need to retrieve books related to the chosen author
 if (queryAuthorValue !== '') {
   parseQuery.equalTo('authors', queryAuthorValue);
 }

 try {
   let books: [Parse.Object] = await parseQuery.find();
   // Many-to-many objects retrieval
   // In this example we need to get every related author Parse.Object
   // and add it to our query result objects
   for (let book of books) {
     // This query is done by creating a relation and querying it
     let bookAuthorsRelation = book.relation('authors');
     book.authorsObjects = await bookAuthorsRelation.query().find();
   }
   setQueriedBooks(books);
   return true;
 } catch (error) {
   // Error can be caused by lack of Internet connection
   Alert.alert('Error!', error.message);
   return false;
 }
};
const queryBooks = async function (): Promise<boolean> {
 // These values come from state variables linked to
 // the screen query RadioButton.Group fields, with its options being every
 // parse object instance saved on server from the referred class, which is
 // queried on screen load via useEffect; these variables retrievie the user choices
 // as a complete Parse.Object;
 const queryPublisherValue: Parse.Object = queryPublisher;
 const queryGenreValue: Parse.Object = queryGenre;
 const queryAuthorValue: Parse.Object = queryAuthor;
 const queryIsbdValue: Parse.Object = queryIsbd;

 // Reading parse objects is done by using Parse.Query
 const parseQuery: Parse.Query = new Parse.Query('Book');

 // One-to-many queries
 if (queryPublisherValue !== '') {
   parseQuery.equalTo('publisher', queryPublisherValue);
 }
 if (queryGenreValue !== '') {
   parseQuery.equalTo('genre', queryGenreValue);
 }

 // One-to-one query
 if (queryIsbdValue !== '') {
   parseQuery.equalTo('isbd', queryIsbdValue);
 }

 // Many-to-many query
 // In this case, we need to retrieve books related to the chosen author
 if (queryAuthorValue !== '') {
   parseQuery.equalTo('authors', queryAuthorValue);
 }

 try {
   let books: [Parse.Object] = await parseQuery.find();
   // Many-to-many objects retrieval
   // In this example we need to get every related author Parse.Object
   // and add it to our query result objects
   for (let book of books) {
     // This query is done by creating a relation and querying it
     let bookAuthorsRelation = book.relation('authors');
     book.authorsObjects = await bookAuthorsRelation.query().find();
   }
   setQueriedBooks(books);
   return true;
 } catch (error) {
   // Error can be caused by lack of Internet connection
   Alert.alert('Error!', error.message);
   return false;
 }
};
﻿
在这种情况下，要查询与特定 ISBD 相关的任何书籍，您只需执行一个 Parse.Query.equalTo 方法，传递 Parse.Object 实例作为参数。查询后，Parse 将在结果对象中存储任何一对一关系字段的完整实例。要从这些对象实例中检索和显示数据，您可以像这样链接 Parse.Object.get 方法： bookParseObject.get(('isbd').get('name'). 下面是使用这些获取器从列表项中检索 ISBD 名称的列表屏幕的样子:
﻿
结论
在本指南的最后，您学习了如何在 React Native 中创建和查询 Parse 的一对一关系。在下一个指南中，我们将向您展示如何注册用户。