EF Core 8关联数据无法加载问题求助

问题场景 #

使用.NET Entity Framework 8.0.1定义了Attraction、Address、Advert三个实体类，其中：

• Attraction与Advert为一对多关系
• Attraction与Address为一对一关系
  已通过迁移创建数据库，但执行查询时Attraction的Address始终为null，Adverts列表始终为空。

实体类代码 #

public class Attraction : BaseEntity
{
    [Required]
    [MaxLength(100)]
    public string Name { get; set; }

    [Required]
    [MaxLength(100)]
    public string Website { get; set; }

    public Address Address { get; set; }

    public ICollection<Advert> Adverts { get; } = new List<Advert>();

    public Attraction()
    {
    }
}

public class Address : BaseEntity
{
    [Required]
    public Guid AttractionId { get; set; }

    [Required]
    [MaxLength(100)]
    public string Line1 { get; set; }

    [Required]
    [MaxLength(100)]
    public string Line2 { get; set; }

    [Required]
    [MaxLength(100)]
    public string Line3 { get; set; }

    [Required]
    [MaxLength(100)]
    public string Line4 { get; set; }

    [Required]
    [MaxLength(100)]
    public string Line5 { get; set; }

    [Required]
    [MaxLength(16)]
    public string PostCode { get; set; }

    public Address()
    {
    }
}

public class Advert : BaseEntity
{
    [Required]
    public Guid AttractionId { get; set; }

    [Required]
    public string Description { get; set; }

    [Required]
    public DateTime DisplayFrom { get; set; }

    [Required]
    public DateTime DisplayTo { get; set; }

    [Required]
    public int DisplayType { get; set; }

    [Required]
    public string PanelColour { get; set; }

    public Advert()
    {
    }
}

基类BaseEntity代码 #

public class BaseEntity
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    public Guid Id { get; set; }
}

DbContext关系配置 #

protected override void OnModelCreating(ModelBuilder modelBuilder)
{          
    modelBuilder.Entity<Attraction>()
        .HasMany(e => e.Adverts)
        .WithOne()
        .HasForeignKey(e => e.AttractionId);

    modelBuilder.Entity<Attraction>()
        .HasOne(e => e.Address)
        .WithOne()
        .HasForeignKey<Address>(e => e.AttractionId);
}

原查询代码 #

public async Task<List<Attraction>> SelectAll()
{
    return await Task.Run(() => AppDbContext.Attractions.ToList());
}

解决方案 #

方法1：使用贪婪加载（Eager Loading）显式加载关联数据 #

EF Core默认不会自动加载导航属性关联的数据，需要通过Include方法指定要加载的关联实体，一次性查询所有需要的数据。同时，EF Core提供了原生异步方法ToListAsync，无需用Task.Run包装异步操作。

修改后的查询代码：

public async Task<List<Attraction>> SelectAll()
{
    return await AppDbContext.Attractions
        .Include(attraction => attraction.Address) // 加载一对一关联的Address
        .Include(attraction => attraction.Adverts) // 加载一对多关联的Adverts集合
        .ToListAsync();
}

方法2：开启延迟加载（Lazy Loading） #

如果希望在访问导航属性时自动加载关联数据，可以开启延迟加载：

1. 安装NuGet包Microsoft.EntityFrameworkCore.Proxies
2. 在DbContext的OnConfiguring方法中启用延迟加载代理：

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder.UseLazyLoadingProxies() // 启用延迟加载代理
                  .UseSqlServer("你的数据库连接字符串");
}

3. 将实体中的导航属性标记为virtual：

public class Attraction : BaseEntity
{
    // 其他属性不变
    public virtual Address Address { get; set; }
    public virtual ICollection<Advert> Adverts { get; } = new List<Advert>();
}

注意：延迟加载可能导致N+1查询问题（先查主实体，再逐个查询关联实体），需根据业务场景评估是否使用。

内容的提问来源于stack exchange，提问作者Adam Messenger