c# - 使用 Entity Framework 进行级联插入

Question

public class User
{
    public int ID { get; set; }

    public string EmailAddress { get; set; }

    public virtual ICollection<Address> Addresses { get; set; }
}

public class Address
{
    public int ID { get; set; }

    public string City { get; set; }

    public string Street { get; set; }

    public string Postcode { get; set; }
}

class TestDbContext : DbContext
{
    public TestDbContext()
        : base("DefaultConnectionString")
    {
    }

    public DbSet<User> Users { get; set; }
    public DbSet<Address> Addresses { get; set; }
}

以上是模型定义和 DbContext 定义。我想为用户添加一个新地址，所以我编写了如下代码:

var context = new TestDbContext();

var user = context.Users.FirstOrDefault(item => item.ID == 1);

user.Addresses.Add(new Address()
{
    City = "City",
    Street = "Street",
    Postcode = "Postcode",
});

context.SaveChanges();

我的疑问是为什么这段代码中执行了3个SQL查询？

1. 它是在 FirstOrDefault 中生成的

   SELECT TOP (1) 
     [Extent1].[ID] AS [ID], 
     [Extent1].[EmailAddress] AS [EmailAddress]
     FROM [dbo].[Users] AS [Extent1]
     WHERE 1 = [Extent1].[ID]
   

2. 它是在 user.Addresses.Add 中生成的

   exec sp_executesql N'SELECT 
       [Extent1].[ID] AS [ID], 
       [Extent1].[City] AS [City], 
       [Extent1].[Street] AS [Street], 
       [Extent1].[Postcode] AS [Postcode], 
       [Extent1].[User_ID] AS [User_ID]
       FROM [dbo].[Addresses] AS [Extent1]
       WHERE ([Extent1].[User_ID] IS NOT NULL) 
         AND ([Extent1].[User_ID] = @EntityKeyValue1)',N'@EntityKeyValue1 int',@EntityKeyValue1=1
   

3. 它是在 SaveChanges 中生成的

   exec sp_executesql N'INSERT [dbo].[Addresses]([City], [Street], [Postcode], [User_ID])
   VALUES (@0, @1, @2, @3)
   SELECT [ID]
   FROM [dbo].[Addresses]
   WHERE @@ROWCOUNT > 0 AND [ID] = scope_identity()',N'@0 nvarchar(max) ,@1 nvarchar(max) ,@2 nvarchar(max) ,@3 int',@0=N'City',@1=N'Street',@2=N'Postcode',@3=1
   

   如何避免第二条 SQL？

Answer 1

当您访问该属性(即 user.Addresses)时，Addresses 导航属性会延迟加载，这就是您获得第二个 SQL 命令的原因。

尝试禁用延迟加载并查看是否有效(不要忘记在 User 的构造函数中初始化 Addresses 属性，例如:

public User()
{
    Addresses = new HashSet<Address>();
}