EF Core中ThenInclude加载Effort.Parent时抛出异常求助

编写了如下EF Core查询代码，尝试通过Include和ThenInclude加载AppUser关联的Signup.Effort.Parent数据：

var user = await context.AppUsers
        .Include(user => user.Following!)
        .ThenInclude(org => org.News)
        .Include(user => user.Interests)
        .Include(user => user.Signups!)
        .ThenInclude(signup => signup.Effort)
        .ThenInclude(effort => effort.Parent)
        .FirstOrDefaultAsync(user => user.Id == _combinedUser.AppUser.Id);

当移除ThenInclude(effort => effort.Parent)时查询正常执行，保留该语句则抛出System.InvalidOperationException异常，异常提示表达式effort.Parent在Include操作中无效，需针对派生类型使用转换。

相关实体类结构如下：

public class Signup : IAuditTrail, IPrimaryEntities, IDisableable
{
    public Effort Effort { get; set; }
    public int EffortId { get; private set; }
}

public abstract class Effort : IAuditTrail, IPrimaryEntities, IDisableable
{
    public Organization Parent { get; set; }
    public int ParentId { get; private set; }
    public List<Signup>? Signups { get; set; }
}

public class Event : Effort
{
}

public class Organization : IPrimaryEntities, IAuditTrail, IDisableable
{
    public List<Event>? Events { get; private set; }
}

原因 #

EF Core无法直接对抽象基类的导航属性使用ThenInclude。因为抽象类本身不会映射到数据库表，实际存储的是它的派生类（此处为Event）。当尝试包含抽象类Effort的Parent属性时，EF Core无法确定具体要加载哪个派生类的关联数据，因此抛出异常要求指定派生类型。

解决方法 #

需要在ThenInclude中通过OfType<派生类>或Cast<派生类>明确指定具体的派生类型，再加载其导航属性。修改后的查询代码如下：

var user = await context.AppUsers
        .Include(user => user.Following!)
        .ThenInclude(org => org.News)
        .Include(user => user.Interests)
        .Include(user => user.Signups!)
        .ThenInclude(signup => signup.Effort.OfType<Event>())
        .ThenInclude(effort => effort.Parent)
        .FirstOrDefaultAsync(user => user.Id == _combinedUser.AppUser.Id);

若所有Effort的实例都是Event，也可以用Cast<Event>：

var user = await context.AppUsers
        .Include(user => user.Following!)
        .ThenInclude(org => org.News)
        .Include(user => user.Interests)
        .Include(user => user.Signups!)
        .ThenInclude(signup => signup.Effort.Cast<Event>())
        .ThenInclude(effort => effort.Parent)
        .FirstOrDefaultAsync(user => user.Id == _combinedUser.AppUser.Id);

这样EF Core就能明确知道要加载Event类型的Parent关联数据，避免异常。

内容的提问来源于stack exchange，提问作者David Thielen