在Entity Framework Core中如何为Address实体加载关联City属性？

问题场景 #

基于ASP.NET 6 + Entity Framework Core + Identity Framework的应用，实体定义如下：

ApplicationUser实体 #

public class ApplicationUser : IdentityUser
{
    public string FullName { get; set; } = string.Empty;
    public bool IsProfessional { get; set; } = false;
    public ICollection<Address>? Addresses { get; set; } = default;
}

Address实体 #

public class Address
{
    public string Id { get; set; } = string.Empty;
    public string UserId { get; set; } = string.Empty;
    public ApplicationUser User { get; set; } = default;
    public string Street { get; set; } = string.Empty;
    public string Number { get; set; } = string.Empty;
    public string Neighborhood { get; set; } = string.Empty;
    public string ZipCode { get; set; } = string.Empty;
    public string CityId { get; set; } = string.Empty;
    public City City { get; set; } = default;
}

City实体 #

public class City
{
    public string Id { get; set; }
    public string Name { get; set; } = string.Empty;
    public State State { get; set; } = default;
    public ICollection<Address>? Addresses { get; set; } = default;
}

在通过UserManager查询用户及其关联地址时，尝试访问Address.City发现该属性为null，查询代码如下：

public async Task<GetUserQueryResponse> Handle(GetUserQuery request, CancellationToken cancellationToken)
{
    var user = await _userManager.Users
        .Include(u => u.Addresses) 
        .Where(u => u.Id == request.UserId).FirstOrDefaultAsync();

    var userAddresses = new List<GetAddressForUserDTO>();
    foreach (var address in user.Addresses)
    {
        userAddresses.Add(new GetAddressForUserDTO()
        {
            Id = address.Id,
            UserId = address.Id, // 此处存在错误，应为address.UserId
            Street = address.Street,
            Number = address.Number,
            Neighborhood = address.Neighborhood,
            ZipCode = address.ZipCode,
            City = address.City.Name, // City为null引发异常
            State = address.City.State.Abbreviation // City为null引发异常
        });
    }

    // 剩余代码
}

问题原因 #

Include(u => u.Addresses)仅加载了用户的地址集合，但并未加载每个地址关联的City实体——EF Core不会自动递归加载所有嵌套关联，必须显式指定需要加载的层级关联。

解决方案 #

1. 使用ThenInclude加载嵌套关联 #

修改查询语句，通过ThenInclude链式加载Address的City；如果需要City关联的State，可以继续链式调用：

var user = await _userManager.Users
    .Include(u => u.Addresses)
        .ThenInclude(a => a.City) // 加载每个Address关联的City
            .ThenInclude(c => c.State) // 如果需要State则添加此行
    .Where(u => u.Id == request.UserId)
    .FirstOrDefaultAsync(cancellationToken);

2. 确保实体外键关系配置正确 #

EF Core可能无法自动识别外键关系，建议在DbContext的OnModelCreating方法中显式配置，避免映射错误：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder); // 必须调用，保留Identity的默认配置

    // 配置Address与City的一对多关系
    modelBuilder.Entity<Address>()
        .HasOne(a => a.City)
        .WithMany(c => c.Addresses)
        .HasForeignKey(a => a.CityId)
        .OnDelete(DeleteBehavior.Restrict); // 根据业务需求设置删除行为
}

3. 空值安全处理 #

由于Addresses、City、State都可能为null，需要添加空值检查避免空引用异常，同时修正原代码中UserId的赋值错误：

var userAddresses = new List<GetAddressForUserDTO>();
if (user?.Addresses != null)
{
    foreach (var address in user.Addresses)
    {
        userAddresses.Add(new GetAddressForUserDTO()
        {
            Id = address.Id,
            UserId = address.UserId, // 修正错误：使用address.UserId而非address.Id
            Street = address.Street,
            Number = address.Number,
            Neighborhood = address.Neighborhood,
            ZipCode = address.ZipCode,
            City = address.City?.Name ?? string.Empty, // 空值合并运算符处理null
            State = address.City?.State?.Abbreviation ?? string.Empty
        });
    }
}

内容的提问来源于stack exchange，提问作者adamasan