EntityFramework的延迟加载不起作用。

Question

我是一名新手，正在学习使用EntityFramework 6，并在POCO方面遇到了问题。

以下是我的代码：

Domain.cs

public abstract class BaseEntity
{
    [Key]
    public long Id { get; set; }
}

public class User : BaseEntity
{
    public long UserRoleId { get; set; }

    public string Name { get; set; }

    public virtual _EntityUserRole UserRole { get; set; }    
}

public class UserRole : BaseEntity
{
    public string Name { get; set; }

    public virtual ICollection<_EntityUser> Users { get; set; }

    public virtual ICollection<_EntityUserPermission> UserPermissions { get; set; }

    internal _EntityUserRole()
    {
        Users = new List<_EntityUser>();
        UserPermissions = new List<_EntityUserPermission>();
    }
}

public class UserPermission : BaseEntity
{
    public string Name { get; set; }

    public virtual ICollection<_EntityUserRole> UserRoles { get; set; }

    internal _EntityUserPermission()
    {
        UserRoles = new List<_EntityUserRole>();
    }
}

Mapping.cs

public abstract class BaseEntityMap<T> : EntityTypeConfiguration<T> where T : BaseEntity
{
    public BaseEntityMap()
    {
        ToTable(typeof(T).Name);
        HasKey(a => a.Id);
    }
}

public class UserMap : BaseEntityMap<User>
{
    public UserMap()
        : base()
    {
        HasRequired(x => x.UserRole)
            .WithMany(x => x.Users)
            .HasForeignKey(x => x.UserRoleId);
    }
}

public class UserRoleMap : BaseEntityMap<UserRole>
{
    public UserRoleMap()
        : base()
    {
        HasMany(x => x.UserPermissions)
            .WithMany(x => x.UserRoles)
            .Map(x =>
            {
                x.MapLeftKey("UserRoleId");
                x.MapRightKey("UserPermissionId");
                x.ToTable("_EntityUserRole_UserPermission");
            });
    }
}

public class UserPermissionMap : BaseEntityMap<UserPermission>
{
    public UserPermissionMap()
        : base()
    {
    }
}

SqlServerDataContext.cs

internal class SqlServerDataContext : DbContext
{
    public DbSet<User> Users { get; set; }

    public DbSet<UserRole> UserRoles { get; set; }

    public DbSet<UserPermission> UserPermissions { get; set; }

    public SqlServerDataContext()
        : base("Name=MyDB")
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new UserMap());
        modelBuilder.Configurations.Add(new UserRoleMap());
        modelBuilder.Configurations.Add(new UserPermissionMap());

        base.OnModelCreating(modelBuilder);
    }
}

Program.cs

    private static void Main()
    {
        using (SqlServerDataContext ctx = new SqlServerDataContext())
        {
            UserRole role = new UserRole();
            role.Name = "Super";

            UserPermission prm1 = new UserPermission();
            prm1.Name = "Delete";
            role.UserPermissions.Add(prm1);

            UserPermission prm2 = new UserPermission();
            prm2.Name = "Add";
            role.UserPermissions.Add(prm2);

            UserPermission prm3 = new UserPermission();
            prm3.Name = "Edit";
            role.UserPermissions.Add(prm3);

            User user = new User();
            user.Name = "User 1";
            user.UserRole = role;

            ctx.Users.Add(user);
            ctx.SaveChanges();
        }

        using (SqlServerDataContext ctx = new SqlServerDataContext())
        {
            var b = ctx.Users.Find(1);
        }
    }

结果

就像在屏幕上打印一样，在我向User插入记录后，当我检索UserPermissions时为空。请帮忙！谢谢。

- Kelvin Go

Answer 1

在将模型添加到集合中后，您需要调用SaveChanges()以便将其持久化到数据库中。

然后，您可以从数据库中读取所需内容。

之后，您需要访问该属性或使用Include（使其成为急切加载，这是首选用法）。

private static void Main()
{
    using (SqlServerDataContext ctx = new SqlServerDataContext())
    {
        UserRole role = new UserRole();
        role.Name = "Super";

        UserPermission prm1 = new UserPermission();
        prm1.Name = "Delete";
        role.UserPermissions.Add(prm1);

        UserPermission prm2 = new UserPermission();
        prm2.Name = "Add";
        role.UserPermissions.Add(prm2);

        UserPermission prm3 = new UserPermission();
        prm3.Name = "Edit";
        role.UserPermissions.Add(prm3);

        ctx.SaveChanges();

        User user = new User();
        user.Name = "User 1";
        user.UserRole = role;

        ctx.Users.Add(user);

        ctx.SaveChanges();
    }

    using (SqlServerDataContext ctx = new SqlServerDataContext())
    {
        var b = ctx.Users.Find(1);

        // Method 1
        var roleName = b.UserRole.Name;

        // Method 2
        ctx.Users.Include(x => x.UserRole).Include(x => x.UserRole.UserPermissions).FirstOrDefault(x => x.Id == 1);
    }
}

- Callum Linington

---

我可以获取UserRole，但无法获取UserPermission。 - Kelvin Go

我更新了答案，我的示例中多了一个 Include！@KelvinGo - Callum Linington

我使用 using System.Data.Entity 添加了代码，但是出现了错误：Error 7 'System.Linq.IQueryable<WinForm.User>' does not contain a definition for 'Find' and no extension method 'Find' accepting a first argument of type 'System.Linq.IQueryable<WinForm.User>' could be found (are you missing a using directive or an assembly reference?) @No1_Melman - Kelvin Go

是的，这是因为 DbSet 具有 Find 方法，但当你开始使用 Include 时，它会将返回类型更改为 IQueryable<T>。因此，你需要使用 FirstOrDefault 查看我的更新 - Callum Linington

@KelvinGo 没问题 :) - Callum Linington