How to: Map a Persistent Class to a Database View Which Has No Key Field如何:映射持久化类到无主键数据库视图

With XAF, you can build new applications from scratch or maintain existing databases. The How to: Generate XPO Business Classes for Existing Data Tables topic describes how to use the design-time wizard that generates business classes for one or more data tables in the specified database at the same time. Additionally, the existing database can contain views (stored queries), which need to be accessed in an XAF application's List Views, Analysis, and Reports. If the database view has a key column, you can map a persistent class to it using the same approach as mapping to a regular table.

使用 XAF，您可以从头开始构建新应用程序或维护现有数据库。"如何：为现有数据表生成 XPO 业务类"主题介绍如何使用设计时向导，该向导同时为指定数据库中的一个或多个数据表生成业务类。此外，现有数据库可以包含需要在 XAF 应用程序的列表视图、分析和报表中访问的视图（存储的查询）。如果数据库视图具有键列，则可以使用与映射到常规表相同的方法将持久类映射到该列。

This topic describes how to implement a persistent class mapped to a database view without a key. We used the "Northwind Traders" demo database, shipped with DXperience Suite and installed in %PUBLIC%\Documents\DevExpress Demos 19.2 \Components\Data\nwind.mdb by default, in this topic. Besides tables filled with demo data, this database includes several database views.

本主题介绍如何实现映射到没有键的数据库视图的持久类。我们使用"北风交易者"演示数据库，在本主题中随 DXperience 套件一起出厂并安装于 %PUBLIC%_文档_DevExpress 演示 19.2 [组件]Datanwind.mdb。本主题中。除了填充演示数据的表外，此数据库还包括多个数据库视图。

Note 注意

This approach uses composite keys, which are not supported in the Mobile applications, and works only for WinForms and ASP.NET applications.

此方法使用移动应用程序中不支持的复合键，并且仅适用于 WinForms 和ASP.NET应用程序。

1.Create a new XAF solution and follow the steps from the How to: Generate XPO Business Classes for Existing Data Tables topic. Do not forget to modify the connection string that connects your application to the nwind.mbd database.

创建新的 XAF 解决方案，并按照"如何：为现有数据表生成 XPO 业务类"主题中的步骤进行操作。不要忘记修改将应用程序连接到 nwind.mbd 数据库的连接字符串。

2.Open the nwind.mbd database to see what views (queries) it contains. You can use Microsoft Office Access or any other MDB viewer application. In this example, the CustomerReports view is mapped to the CustomerReports persistent class:

打开 nwind.mbd 数据库以查看它包含哪些视图（查询）。您可以使用 Microsoft Office 访问或任何其他 MDB 查看器应用程序。在此示例中，客户报告视图映射到客户报告持久性类：

Note 注意

This view contains the ProductName, CompanyName, OrderDate and ProductAmount fields. These names are used when implementing the CustomerReports class.

此视图包含"产品名称、公司名称、订单日期"和"产品金额"字段。在实现客户报告类时使用这些名称。

3.Create a new CustomerReports persistent class (you can use the DevExpress 19.2 ORM Persistent Object template). Replace the automatically generated class declaration with the following code:

创建新的客户报告持久性类（您可以使用 DevExpress 19.2 ORM 持久对象模板）。将自动生成的类声明替换为以下代码：

using DevExpress.ExpressApp.Xpo.Utils;
using DevExpress.Persistent.Base;
using DevExpress.Xpo;
// ...
[DefaultClassOptions]
public class CustomerReports : XPLiteObject {
    public CustomerReports(Session session) : base(session) { }
    CustomerReportsViewKey fKey;
    [Key, Persistent, Browsable(false)]
    public CustomerReportsViewKey Key {
        get { return fKey; }
        set { SetPropertyValue(nameof(Key), ref fKey, value); }
    }
    public string ProductName { get { return Key.ProductName; } }
    public string CompanyName { get { return Key.CompanyName; } }
    public DateTime OrderDate { get { return Key.OrderDate; } }
    public string ProductAmount { get { return Key.ProductAmount; } }
}
[TypeConverter(typeof(StructTypeConverter<CustomerReportsViewKey>))]
public struct CustomerReportsViewKey {
    string fProductName;
    [Persistent("ProductName"), Browsable(false)]
    public string ProductName {
        get { return fProductName; }
        set { SetPropertyValue(nameof(ProductName), ref fProductName, value); }
    }
    string fCompanyName;
    [Persistent("CompanyName"), Browsable(false)]
    public string CompanyName {
        get { return fCompanyName; }
        set { SetPropertyValue(nameof(CompanyName), ref fCompanyName, value); }
    }
    DateTime fOrderDate;
    [Persistent("OrderDate"), Browsable(false)]
    public DateTime OrderDate {
        get { return fOrderDate; }
        set { SetPropertyValue(nameof(OrderDate), ref fOrderDate, value); }
    }
    string fProductAmount;
    [Persistent("ProductAmount"), Browsable(false)]
    public string ProductAmount {
        get { return fProductAmount; }
        set { SetPropertyValue(nameof(ProductAmount), ref fProductAmount, value); }
    }
}

Each persistent class requires a primary key. The CustomerReports class is an XPLiteObject class descendant, which has no auto-generated key property. So, the Key property representing a composite key (a key formed by combining at least two or more columns) was implemented.

每个持久类都需要一个主键。CustomerReports 类是 XPLiteObject 类后代，没有自动生成的密钥属性。因此，实现了表示复合键的 Key 属性（通过组合至少两个或多个列而形成的键）。

The CustomerReportsViewKey struct defines columns which form a composite key. The struct requires the TypeConverter attribute, which enables ASPxGridView to recognize the key value and to process objects.

客户报告视图密钥结构定义构成复合键的列。结构需要 TypeConverter 属性，该属性使 ASPxGridView 能够识别键值并处理对象。

Additionally, the CustomerReports class exposes properties corresponding to all the view's columns. You can omit any property, but the composite key should still include all columns.

此外，CustomerReports 类公开与所有视图列对应的属性。可以省略任何属性，但复合键仍应包含所有列。

Note 注意

• A database view can already have a key column. In this case, you do not need the composite key, and you should decorate the key property with the KeyAttribute.

• The number of columns included in the composite key is limited. For instance, Microsoft SQL Server allows a maximum of 16 columns.

• 数据库视图可以具有键列。在这种情况下，不需要复合键，并且应该使用 KeyAttribute 装饰该键属性。

• 复合键中包含的列数受到限制。例如，Microsoft SQL Server 最多允许 16 列。

If you do not want your class to have the same name as the database view, you can use a custom name, and decorate the class with the PersistentAttribute:

如果不希望类具有与数据库视图相同的名称，则可以使用自定义名称，并使用持久属性装饰类：

[DefaultClassOptions, Persistent("CustomerReports")]
public class MyCustomerReports : XPLiteObject {
    // ...
}

4.Run the application. The "Customer Reports" object is available.

运行应用程序。"客户报告"对象可用。

You can use the CustomerReports class as the Data Type in Reports V2 and Analysis:

您可以将客户报告类用作报表 V2 和分析中的数据类型：