LINQ to SQL语句(12)之Delete和使用Attach

Posted 2020-08-24 可乐•雪碧•咖啡•茶

1.简单形式

说明：调用DeleteOnSubmit方法即可。

OrderDetail orderDetail =
    db.OrderDetails.First
    (c => c.OrderID == 10255 && c.ProductID == 36);
db.OrderDetails.DeleteOnSubmit(orderDetail);
db.SubmitChanges();

语句描述：使用DeleteOnSubmit方法从OrderDetail 表中删除OrderDetail对象。调用SubmitChanges 将此删除保持到数据库。

2.一对多关系

说明：Order与OrderDetail是一对多关系，首先DeleteOnSubmit其OrderDetail(多端)，其次DeleteOnSubmit其Order(一端)。因为一端是主键。

var orderDetails =
    from o in db.OrderDetails
    where o.Order.CustomerID == "WARTH" &&
    o.Order.EmployeeID == 3
    select o;
var order =
    (from o in db.Orders
     where o.CustomerID == "WARTH" && o.EmployeeID == 3
     select o).First();
foreach (OrderDetail od in orderDetails)
{
    db.OrderDetails.DeleteOnSubmit(od);
}
db.Orders.DeleteOnSubmit(order);
db.SubmitChanges();

语句描述语句描述：使用DeleteOnSubmit方法从Order 和Order Details表中删除Order和Order Detail对象。首先从Order Details删除，然后从Orders删除。调用SubmitChanges将此删除保持到数据库。

3.推理删除(Inferred Delete)

说明：Order与OrderDetail是一对多关系，在上面的例子，我们全部删除CustomerID为WARTH和EmployeeID为3 的数据，那么我们不须全部删除呢？例如Order的OrderID为10248的OrderDetail有很多，但是我们只要删除ProductID为11的OrderDetail。这时就用Remove方法。

Order order = db.Orders.First(x => x.OrderID == 10248);
OrderDetail od = 
    order.OrderDetails.First(d => d.ProductID == 11);
order.OrderDetails.Remove(od);
db.SubmitChanges();

语句描述语句描述：这个例子说明在实体对象的引用实体将该对象从其EntitySet 中移除时，推理删除如何导致在该对象上发生实际的删除操作。仅当实体的关联映射将DeleteOnNull设置为true且CanBeNull 为false 时，才会发生推理删除行为。  

使用Attach更新(Update with Attach)

说明：在对于在不同的DataContext之间，使用Attach方法来更新数据。例如在一个名为tempdb的NorthwindDataContext中，查询出Customer和Order，在另一个NorthwindDataContext中，Customer的地址更新为123 First Ave，Order的CustomerID 更新为CHOPS。

//通常，通过从其他层反序列化 XML 来获取要附加的实体
//不支持将实体从一个DataContext附加到另一个DataContext
//因此若要复制反序列化实体的操作，将在此处重新创建这些实体
Customer c1;
List<Order> deserializedOrders = new List<Order>();
Customer deserializedC1;
using (NorthwindDataContext tempdb = new NorthwindDataContext())
{
    c1 = tempdb.Customers.Single(c => c.CustomerID == "ALFKI");
    deserializedC1 = new Customer
    {
        Address = c1.Address,
        City = c1.City,
        CompanyName = c1.CompanyName,
        ContactName = c1.ContactName,
        ContactTitle = c1.ContactTitle,
        Country = c1.Country,
        CustomerID = c1.CustomerID,
        Fax = c1.Fax,
        Phone = c1.Phone,
        PostalCode = c1.PostalCode,
        Region = c1.Region
    };
    Customer tempcust =
        tempdb.Customers.Single(c => c.CustomerID == "ANTON");
    foreach (Order o in tempcust.Orders)
    {
        deserializedOrders.Add(new Order
        {
            CustomerID = o.CustomerID,
            EmployeeID = o.EmployeeID,
            Freight = o.Freight,
            OrderDate = o.OrderDate,
            OrderID = o.OrderID,
            RequiredDate = o.RequiredDate,
            ShipAddress = o.ShipAddress,
            ShipCity = o.ShipCity,
            ShipName = o.ShipName,
            ShipCountry = o.ShipCountry,
            ShippedDate = o.ShippedDate,
            ShipPostalCode = o.ShipPostalCode,
            ShipRegion = o.ShipRegion,
            ShipVia = o.ShipVia
        });
    }
}
using (NorthwindDataContext db2 = new NorthwindDataContext())
{
    //将第一个实体附加到当前数据上下文，以跟踪更改
    //对Customer更新，不能写错
    db2.Customers.Attach(deserializedC1);
    //更改所跟踪的实体
    deserializedC1.Address = "123 First Ave";
    //附加订单列表中的所有实体
    db2.Orders.AttachAll(deserializedOrders);
    //将订单更新为属于其他客户
    foreach (Order o in deserializedOrders)
    {
        o.CustomerID = "CHOPS";
    }
    //在当前数据上下文中提交更改
    db2.SubmitChanges();
}

语句描述：从另一个层中获取实体，使用Attach和AttachAll将反序列化后的实体附加到数据上下文，然后更新实体。更改被提交到数据库。

使用Attach更新和删除(Update and Delete with Attach)

说明：在不同的DataContext中，实现插入、更新、删除。看下面的一个例子：

//通常，通过从其他层反序列化XML获取要附加的实体
//此示例使用 LoadWith 在一个查询中预先加载客户和订单，
//并禁用延迟加载
Customer cust = null;
using (NorthwindDataContext tempdb = new NorthwindDataContext())
{
    DataLoadOptions shape = new DataLoadOptions();
    shape.LoadWith<Customer>(c => c.Orders);
    //加载第一个客户实体及其订单
    tempdb.LoadOptions = shape;
    tempdb.DeferredLoadingEnabled = false;
    cust = tempdb.Customers.First(x => x.CustomerID == "ALFKI");
}
Order orderA = cust.Orders.First();
Order orderB = cust.Orders.First(x => x.OrderID > orderA.OrderID);
using (NorthwindDataContext db2 = new NorthwindDataContext())
{
    //将第一个实体附加到当前数据上下文，以跟踪更改
    db2.Customers.Attach(cust);
    //附加相关订单以进行跟踪; 否则将在提交时插入它们
    db2.Orders.AttachAll(cust.Orders.ToList());
    //更新客户的Phone.
    cust.Phone = "2345 5436";
    //更新第一个订单OrderA的ShipCity.
    orderA.ShipCity = "Redmond";
    //移除第二个订单OrderB.
    cust.Orders.Remove(orderB);
    //添加一个新的订单Order到客户Customer中.
    Order orderC = new Order() { ShipCity = "New York" };
    cust.Orders.Add(orderC);
    //提交执行
    db2.SubmitChanges();
}

语句描述：从一个上下文提取实体，并使用 Attach 和 AttachAll 附加来自其他上下文的实体，然后更新这两个实体，删除一个实体，添加另一个实体。更改被提交到数据库。