查詢實體和關聯 (Entity Framework 快速入門)

發行項
02/28/2011

在這項工作中，您將針對代表 School 模型中之實體 (Entity) 和關聯的 CLR 物件建立強型別 (Strongly Typed) 查詢，並且將顯示控制項繫結至從這些查詢傳回的物件集合。

查詢 School 資料庫中的部門

在 CourseViewer 表單的程式碼檔案開頭，加入下列 using (C#) 或 Imports (Visual Basic) 陳述式 (Statement)，以便參考根據 School 資料庫所建立的模型以及實體命名空間 (Namespace)。
```
Imports System.Data.Objects
Imports System.Data.Objects.DataClasses
```
```
using System.Data.Objects;
using System.Data.Objects.DataClasses;
```

在 CourseViewer 表單的部分類別定義頂端，加入下列可建立 ObjectContext 執行個體 (Instance) 的程式碼。

' Create an ObjectContext instance based on SchoolEntity.
Private schoolContext As SchoolEntities

//Create an ObjectContext instance based on SchoolEntity
private SchoolEntities schoolContext;

在 CourseViewer 表單設計工具中，按兩下 CourseViewer 表單。

這樣會開啟表單的字碼頁 (Code Page) 並且建立 courseViewer _Load 事件處理常式方法。

在 courseViewer _Load 事件處理常式方中，複製並貼上下列可定義 DataGridView 的程式碼、執行可傳回部門集合 (按照 Name 排列順序) 的查詢，然後將 Department 物件的集合繫結至 departmentList 控制項。

' 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) = _
    From d In schoolContext.Departments.Include("Courses") _
    Order By d.Name _
    Select d
Try
    ' Bind the ComboBox control to the query.
    ' To prevent the query from being executed multiple times during binding, 
    ' it is recommended to bind controls to the result of the Execute method. 
    Me.departmentList.DisplayMember = "Name"
    Me.departmentList.DataSource = CType(departmentQuery, ObjectQuery).Execute(MergeOption.AppendOnly)
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 course objects, ordered by name.
var departmentQuery = from d in schoolContext.Departments.Include("Courses")
                      orderby d.Name
                      select d;
try
{
    // Bind the ComboBox control to the query, 
    // which is executed during data binding.
    // To prevent the query from being executed multiple times during binding, 
    // it is recommended to bind controls to the result of the Execute method. 
    this.departmentList.DisplayMember = "Name";
    this.departmentList.DataSource = ((ObjectQuery)departmentQuery).Execute(MergeOption.AppendOnly);
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message);
}

顯示所選取部門的課程

在 CourseViewer 表單設計工具中，按兩下 departmentList 控制項。

這樣會建立 departmentList_SelectedIndexChanged 事件處理常式方法。

貼上下列用來載入與所選取部門相關之課程的程式碼。

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.Courses

    ' Hide the columns that are bound to the navigation properties on Course.
    courseGridView.Columns("Department").Visible = False
    courseGridView.Columns("StudentGrades").Visible = False
    courseGridView.Columns("OnlineCourse").Visible = False
    courseGridView.Columns("OnsiteCourse").Visible = False
    courseGridView.Columns("People").Visible = False
    courseGridView.Columns("DepartmentId").Visible = False

    courseGridView.AllowUserToDeleteRows = False
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.Courses;

        // Hide the columns that are bound to the navigation properties on Course.
        courseGridView.Columns["Department"].Visible = false;
        courseGridView.Columns["StudentGrades"].Visible = false;
        courseGridView.Columns["OnlineCourse"].Visible = false;
        courseGridView.Columns["OnsiteCourse"].Visible = false;
        courseGridView.Columns["People"].Visible = false;
        courseGridView.Columns["DepartmentId"].Visible = false;

        courseGridView.AllowUserToDeleteRows = false;
courseGridView.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }

後續步驟

您已成功建立可傳回 Department 和 Course 物件的查詢，並且將這些物件繫結至控制項。接下來，請將您對資料格中 Course 物件做的變更儲存回資料庫：插入及更新資料 (Entity Framework 快速入門)。

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查詢實體和關聯 (Entity Framework 快速入門)

查詢 School 資料庫中的部門

顯示所選取部門的課程

後續步驟

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