React Native 中 Parse SDK 的多对多数据关联指南

8min
多对多关系
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。这可以在构建和运行复杂查询时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；
﻿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，我们将重点关注多对多关系。这些关系在包含创作内容的应用程序中很常见，例如博客和新闻源，因为作者和类别标签可以互换并优化以快速查询。数据存储后端通常会要求明确声明这些关联，甚至要求您自己创建关系表以管理它们的连接。Parse为您自动完成这一切，消除了在构建数据关系时出错的可能性，并加快了您的应用程序建模过程。
在Parse中创建多对多关系有两种方法。第一种使用Parse.Object 关系，这在创建和查询时间上是最快的。第二种是使用数组，这可能会导致查询时间变慢，具体取决于它们的大小。由于这个性能问题，从现在开始我们将仅使用关系示例。
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创建和查询多对多数据关系。
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﻿
先决条件
要完成本教程，您需要：
一个创建的 React Native 应用程序，并且 连接到 Back4App.
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 React Native 中使用 Parse 在现实场景中执行和演示多对多数据库关系。
- 理解书籍类
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：
﻿出版社: 书籍出版社名称，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。
这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 对于 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，因此当用户选择出版商或类型选项时，表单字段将包含完整的 Parse.Object 实例。
﻿
让我们来看看提交此表单时调用的书籍创建方法：
JavaScript
TodoList.tsx
const createBook = async function (): Promise<boolean> {
 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: string = bookTitle;
   const bookISBDValue: string = 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: Parse.Object = bookPublisher;
   const bookGenreObject: Parse.Object = bookGenre;
   // bookAuthors can be an array of Parse.Objects, since the book
   // may have more than one Author
   const bookAuthorsObjects: [Parse.Object] = bookAuthors;

   // Creates a new parse object instance
   let Book: Parse.Object = 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: Parse.Query = new Parse.Query('ISBD');
   isbdQuery.equalTo('name', bookISBDValue);
   let isbdQueryResult: Parse.Object = 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: Parse.Object = 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 (): Promise<boolean> {
 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: string = bookTitle;
   const bookISBDValue: string = 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: Parse.Object = bookPublisher;
   const bookGenreObject: Parse.Object = bookGenre;
   // bookAuthors can be an array of Parse.Objects, since the book
   // may have more than one Author
   const bookAuthorsObjects: [Parse.Object] = bookAuthors;

   // Creates a new parse object instance
   let Book: Parse.Object = 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: Parse.Query = new Parse.Query('ISBD');
   isbdQuery.equalTo('name', bookISBDValue);
   let isbdQueryResult: Parse.Object = 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: Parse.Object = 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 bookAuthorsObjects 是如何设置为新的书籍 Parse.Object 实例的。要创建多对多关系，您首先需要创建一个新的 Parse.Object.relation，然后将相关对象逐个添加到其中，或者通过传递一个 Parse.Object 的数组来添加，使用 Parse.Object.relation.add 方法。Parse 将在您的数据库中创建一个关系类型列，并且还会创建一个关系表。Parse 还将在仪表板的字段列中为方便访问此新表创建一个链接。
﻿
请注意，一旦创建了仪表板，您还可以管理对象并向您的关系中添加新对象，因此在开发应用程序时请记住这个快捷方式。
- 查询多对多关系
从多对多相关对象查询中检索结果需要一些额外的步骤，与常规查询相比。请查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，要查询与特定作者相关的任何书籍，您需要执行一个 Parse.Query.equalTo 方法，传递 Parse.Object 实例作为参数。查询后，Parse 不会存储来自多对多关系字段的 Parse.Object 实例，只会存储对相关类名的引用，例如 {"__type": "Relation", "className": "Author"}. 要从这些对象实例中检索和显示数据，您需要创建一个关系并再次查询，将结果存储在您自己的对象数组中。之后，您可以遍历对象数组并使用 Parse.Object.get 方法来检索作者的名字。以下是使用这些程序的列表屏幕的样子:
﻿
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 的多对多关系。在下一个指南中，我们将向您展示如何进行一对一查询和关系。