查询实体和关联(实体框架快速入门)
在本任务中,您将针对在 School 模型中表示实体和关联的 CLR 对象创建强类型查询,并将显示控件绑定到从这些查询中返回的对象集合。
在 School 数据库中查询系
在 CourseViewer 窗体的代码文件的开始处,添加以下 using (C#) 或 Imports (Visual Basic) 语句,以引用从 School 数据库和实体命名空间中创建的模型。
Imports System.Data.Objects Imports System.Data.Objects.DataClasses
using System.Data.Objects; using System.Data.Objects.DataClasses;
在 CourseViewer 窗体的分部类定义的顶部,添加以下用于创建 ObjectContext 实例的代码。
' Create an ObjectContext instance based on SchoolEntity. Private schoolContext As SchoolEntities
// Create an ObjectContext instance based on SchoolEntity. private SchoolEntities schoolContext;
在 CourseViewer 窗体设计器中,双击 CourseViewer 窗体。
此时将打开窗体的代码页并创建 courseViewer _Load 事件处理程序方法。
在 courseViewer _Load 事件处理程序方法中,复制并粘贴以下用于定义 DataGridView 的代码,执行返回系科集合的查询(按 Name 排序),然后将 Department 对象的集合绑定到 departmentList 控件。
' Initialize the ObjectContext. schoolContext = New SchoolEntities() ' Define a query that returns all Department objects and related ' Course objects, ordered by name. Dim departmentQuery As ObjectQuery(Of Department) = _ schoolContext.Department.Include("Course").OrderBy("it.Name") Try ' Bind the ComboBox control to the query, which is ' executed during data binding. Me.departmentList.DataSource = departmentQuery Me.departmentList.DisplayMember = "Name" Catch ex As Exception MessageBox.Show(ex.Message) End Try
// Initialize the ObjectContext. schoolContext = new SchoolEntities(); // Define a query that returns all Department objects and related // Course objects, ordered by name. ObjectQuery<Department> departmentQuery = schoolContext.Department.Include("Course").OrderBy("it.Name"); try { // Bind the ComboBox control to the query, which is // executed during data binding. this.departmentList.DataSource = departmentQuery; this.departmentList.DisplayMember = "Name"; } catch (Exception ex) { MessageBox.Show(ex.Message); }
显示所选系的课程
在 CourseViewer 窗体设计器中,双击 departmentList 控件。
此时将创建 departmentList_SelectedIndexChanged 事件处理程序方法。
粘贴以下用于加载与所选系相关的课程的代码。
Try ' Get the object for the selected department. Dim department As Department = _ CType(Me.departmentList.SelectedItem, Department) ' Bind the grid view to the collection of Course objects ' that are related to the selected Department object. courseGridView.DataSource = department.Course courseGridView.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells) Catch ex As Exception MessageBox.Show(ex.Message) End Try
try { // Get the object for the selected department. Department department = (Department)this.departmentList.SelectedItem; // Bind the grid view to the collection of Course objects // that are related to the selected Department object. courseGridView.DataSource = department.Course; courseGridView.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells); } catch (Exception ex) { MessageBox.Show(ex.Message); }
后续步骤
您已成功地创建了返回 Department 和 Course 对象的查询并将这些对象绑定到控件。接下来,您需要将在数据网格中对 Course 对象所做的更改保存到数据库中:插入和更新数据(实体框架快速入门)。