设置属性时实体框架的延迟加载

Question

我有一个使用实体框架查询数据库并返回EF对象（ApplicationUser）的方法。 如果我有一个ApplicationUser的构造函数，尝试使用此方法填充其属性，那么设置该对象时，该对象的复杂属性会触发数据库被调用吗？ 在原始对象中，它们是代理，因此将被延迟加载，但是仅将那些属性设置为同一对象的新实例的属性会调用数据库吗？

这是一个简单的例子：

public class ApplicationUser
{
    public ApplicationUser(int userId)
    {
        ApplicationUser user = ApplicationUser.Select(userId);
        this.FirstName = user.FirstName //this is fine
        this.ApplicationRole = user.ApplicationRole //This is a complex object. Is it still going to be lazily loaded and called only if this.ApplicationRole is referenced?
    }

    public static ApplicationUser Select(int userId)
    {
        //this uses EF to return an Application User object
    }
}

Answer 1

数据库将在以下行中调用：

this.ApplicationRole = user.ApplicationRole;

然后，将获取您的对象，并且不再延迟加载。

为确保发生这种情况，您必须进行一些配置：

1. 必须启用延迟加载和代理创建：

    public Context() : base("YourConnectionString") { Configuration.LazyLoadingEnabled = true; Configuration.ProxyCreationEnabled = true; } 

2. 导航属性必须声明为virutal ：

    public class ApplicationUser { //... public virtual ApplicationRole ApplicationRole { get; set; } //... } 

实际例子

假设您有这些课程

public class ApplicationUser
{
    public int ApplicationUserId { get; set; }
    public string FirstName { get; set; }
    public virtual ApplicationRole ApplicationRole { get; set; }

    public static Context db { get; set; }

    public ApplicationUser()
    {

    }

    public ApplicationUser(int userId)
    {
        ApplicationUser user = ApplicationUser.Select(userId);
        this.FirstName = user.FirstName; 
        this.ApplicationRole = user.ApplicationRole; //database will be hit here
    }

    public static ApplicationUser Select(int userId)
    {
        //this uses EF to return an Application User object
        return db.Users.Find(userId);
    }
}

public class ApplicationRole
{
    public int ApplicationRoleId { get; set;}
    public string Name { get; set; }
}

第一击：

return db.Users.Find(userId);

生成的SQL：

SELECT
    [Limit1].[ApplicationUserId] AS [ApplicationUserId],
    [Limit1].[FirstName] AS [FirstName],
    [Limit1].[ApplicationRole_ApplicationRoleId] AS [ApplicationRole_Application
RoleId]
    FROM ( SELECT TOP (2)
        [Extent1].[ApplicationUserId] AS [ApplicationUserId],
        [Extent1].[FirstName] AS [FirstName],
        [Extent1].[ApplicationRole_ApplicationRoleId] AS [ApplicationRole_Applic
ationRoleId]
        FROM [dbo].[ApplicationUsers] AS [Extent1]
        WHERE [Extent1].[ApplicationUserId] = @p0
    )  AS [Limit1]

-- p0: '1' (Type = Int32)

第二击

this.ApplicationRole = user.ApplicationRole

生成的SQL

SELECT
    [Extent2].[ApplicationRoleId] AS [ApplicationRoleId],
    [Extent2].[Name] AS [Name]
    FROM  [dbo].[ApplicationUsers] AS [Extent1]
    INNER JOIN [dbo].[ApplicationRoles] AS [Extent2] ON [Extent1].[ApplicationRo
le_ApplicationRoleId] = [Extent2].[ApplicationRoleId]
    WHERE ([Extent1].[ApplicationRole_ApplicationRoleId] IS NOT NULL) AND ([Exte
nt1].[ApplicationUserId] = @EntityKeyValue1)


-- EntityKeyValue1: '1' (Type = Int32, IsNullable = false)

如果ApplicationRole对象具有其他导航属性，则它们仍将被延迟加载，而不会出现问题。

---

这里有一个提示可能会有所帮助。 要查看EF何时访问数据库，请创建一个控制台应用程序，添加解决方案的引用，然后使用context.Database.Log 。 要查看有关日志记录的更多信息，请查看此链接https://msdn.microsoft.com/zh-cn/data/dn469464.aspx