如何：显式加载相关对象（实体框架）

本主题提供有关如何显式加载相关对象的示例。

第一个示例使用 EntityCollection 的 Load 方法来显式加载单个客户的所有订单和项。 由于 POCO 实体的导航属性不需要返回 EntityCollection，因此无法将 Load 方法与 POCO 实体一起使用。 有关更多信息，请参见加载相关 POCO 实体（实体框架）。

第二个示例使用 CreateSourceQuery 方法创建一个查询，此查询只加载包含相关项的选定订单。 然后将这些订单附加到该客户。

您还可以使用 ObjectContext 类的 LoadProperty 方法加载相关对象。 LoadProperty 方法可与 POCO 实体和派生自 EntityObject 的实体一起使用。

本主题中的示例基于 Adventure Works 销售模型。 有关更多信息，请参见如何：使用实体数据模型向导（实体框架）。

示例

下面的示例加载属于单个 Contact 的 SalesOrderHeader 对象，然后循环访问 EntityCollection 中的 SalesOrderHeader 对象。 对于此集合中的每个 SalesOrderHeader 对象，调用 Load 方法以从数据库检索相关的 SalesOrderDetail 对象的集合。

' Specify the customer ID. 
Dim contactID As Integer = 4332

Using context As New AdventureWorksEntities()
    context.ContextOptions.LazyLoadingEnabled = False

    ' Get a specified customer by contact ID. 
    Dim contact = (From c In context.Contacts
        Where c.ContactID = contactID
        Select c).First()

    ' Load the orders for the customer explicitly. 
    If Not contact.SalesOrderHeaders.IsLoaded Then
        contact.SalesOrderHeaders.Load()
    End If

    For Each order As SalesOrderHeader In contact.SalesOrderHeaders
        ' Load the items for the order if not already loaded. 
        If Not order.SalesOrderDetails.IsLoaded Then
            order.SalesOrderDetails.Load()
        End If

        Console.WriteLine(String.Format("PO Number: {0}", order.PurchaseOrderNumber))
        Console.WriteLine(String.Format("Order Date: {0}", order.OrderDate.ToString()))
        Console.WriteLine("Order items:")
        For Each item As SalesOrderDetail In order.SalesOrderDetails
            Console.WriteLine(String.Format("Product: {0} Quantity: {1}", _
                                              item.ProductID.ToString(), item.OrderQty.ToString()))
        Next
    Next
End Using

// Specify the customer ID.
int contactID = 4332;

using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    context.ContextOptions.LazyLoadingEnabled = false;

    // Get a specified customer by contact ID.
    var contact =
        (from c in context.Contacts
         where c.ContactID == contactID
         select c).First();

    // Load the orders for the customer explicitly.
    if (!contact.SalesOrderHeaders.IsLoaded)
    {
        contact.SalesOrderHeaders.Load();
    }

    foreach (SalesOrderHeader order in contact.SalesOrderHeaders)
    {
        // Load the items for the order if not already loaded.
        if (!order.SalesOrderDetails.IsLoaded)
        {
            order.SalesOrderDetails.Load();
        }

        Console.WriteLine(String.Format("PO Number: {0}",
            order.PurchaseOrderNumber));
        Console.WriteLine(String.Format("Order Date: {0}",
            order.OrderDate.ToString()));
        Console.WriteLine("Order items:");
        foreach (SalesOrderDetail item in order.SalesOrderDetails)
        {
            Console.WriteLine(String.Format("Product: {0} "
                + "Quantity: {1}", item.ProductID.ToString(),
                item.OrderQty.ToString()));
        }
    }
}

下面的示例对 EntityCollection 使用 CreateSourceQuery 方法，以仅加载五个具有相关 SalesOrderDetail 对象（这些对象属于单个 Contact）的 SalesOrderHeader 对象。 随后，此查询结果会附加到原始 SalesOrderHeader EntityCollection。

' Specify the customer ID. 
Dim customerId As Integer = 4332

Using context As New AdventureWorksEntities()
    ' Get a specified customer by contact ID. 
    Dim customer As Contact = context.Contacts.Where("it.ContactID = @customerId", _
                                                     New ObjectParameter("customerId", customerId)).First()

    ' Return the customer's first five orders with line items and 
    ' attach them to the SalesOrderHeader collection. 
    customer.SalesOrderHeaders.Attach(customer.SalesOrderHeaders.CreateSourceQuery().Include("SalesOrderDetails").Take(5))

    For Each order As SalesOrderHeader In customer.SalesOrderHeaders
        Console.WriteLine(String.Format("PO Number: {0}", order.PurchaseOrderNumber))
        Console.WriteLine(String.Format("Order Date: {0}", order.OrderDate.ToString()))
        Console.WriteLine("Order items:")

        For Each item As SalesOrderDetail In order.SalesOrderDetails
            Console.WriteLine(String.Format("Product: {0} Quantity: {1}", _
                                              item.ProductID.ToString(), item.OrderQty.ToString()))
        Next
    Next
End Using

// Specify the customer ID.
int customerId = 4332;

using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    // Get a specified customer by contact ID.
    Contact customer = context.Contacts
        .Where("it.ContactID = @customerId",
        new ObjectParameter("customerId", customerId)).First();

    // Return the customer's first five orders with line items and
    // attach them to the SalesOrderHeader collection.
    customer.SalesOrderHeaders.Attach(
        customer.SalesOrderHeaders.CreateSourceQuery()
        .Include("SalesOrderDetails").Take(5));

    foreach (SalesOrderHeader order in customer.SalesOrderHeaders)
    {
        Console.WriteLine(String.Format("PO Number: {0}",
            order.PurchaseOrderNumber));
        Console.WriteLine(String.Format("Order Date: {0}",
            order.OrderDate.ToString()));
        Console.WriteLine("Order items:");

        foreach (SalesOrderDetail item in order.SalesOrderDetails)
        {
            Console.WriteLine(String.Format("Product: {0} "
                + "Quantity: {1}", item.ProductID.ToString(),
                item.OrderQty.ToString()));
        }
    }
}

另请参见

任务

如何：执行返回实体类型对象的查询（实体框架）
如何：使用查询路径调整结果（实体框架）
如何：使用导航属性导航关系（实体框架）

概念

加载相关对象（实体框架）