具有静态实例成员的实体查询条件

Question

我正在尝试创建一个类似于订单状态历史表的审计跟踪。这样，Orders 可以有许多 OrderState，以及一个指向最新历史项目的 State。保存更新时到目前为止一切顺利。当我尝试像使用枚举一样查询时会出现问题：

public class OrderState
{
    public static OrderState Placed = new OrderState("Placed", 1, 1);
    public static OrderState Accepted = new OrderState("Accepted", 10, 2);
    public static OrderState Cancelled = new OrderState("Cancelled", 20, 3);
    public static OrderState Completed = new OrderState("Completed", 30, 4);

    protected OrderState()
    {
    }

    public OrderState(string name, int order, int id)
    {
        Name = name;
        Order = order;
        Id = id;
    }

    public int Id { get; set; }
    public string Name { get; protected set; }
    public int Order { get; protected set; }



    public static bool operator == (OrderState state1, OrderState state2)
    {
        if (ReferenceEquals(state1, null))
        {
            return ReferenceEquals(state2, null);
        }
        return state1.Equals(state2);
    }

    public static bool operator !=(OrderState state1, OrderState state2)
    {
        return !(state1 == state2);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj))
        {
            return false;
        }
        if (ReferenceEquals(this, obj))
        {
            return true;
        }
        if (!(obj is OrderState))
        {
            return false;
        }
        return Equals((OrderState)obj);
    }

    public virtual bool Equals(OrderState other)
    {
        return other.Id.Equals(Id);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return ((Id.GetHashCode())*397) ^ Order;
        }
    }


}

订单类

public class Order
{
    public Order()
    {
        Progress(OrderState.Placed);
    }
    public int Id { get; set; }

    public virtual OrderState State
    {
        get { return States.OrderByDescending(x => x.State.Order).FirstOrDefault()?.State; }
    }

    public void Progress(OrderState state)
    {
        if (States.All(x => x.State != state))
        {
            States.Add(new OrderStateHistory()
            {
                Order = this,
                State = state
            });
        }
    }

    public virtual ICollection<OrderStateHistory> States { get; set; } = new List<OrderStateHistory>();
}

在我的代码中，这样的事情可以正常工作： order.Progress(OrderState.Accepted);，if (order.State == OrderState.Accepted)

但是，我想访问的是 Where(x => x.State.Equals(OrderState.Accepted)) 或 Where(x => x.State == OrderState.Accepted)

不幸的是，任何一个条件都会产生“LINQ to Entities 不支持指定类型成员“状态”。仅支持初始化程序、实体成员和实体导航属性。错误。

我知道我已经能够使用 NHibernate 做到这一点。我什至可以用 EF 做到这一点吗？

Answer 1

由于 EF 需要将您的 LINQ 语句转换为 SQL 语句， 你不能这样做。如果您有复杂的比较逻辑 overridden Equals() 方法，您必须在 LINQ 语句。

public IQueryable<Foo> FoosEqualTo(IQueryable<Foo> allFoos, Foo target) {
return from foo in allFoos
       where foo.Id == target.Id // or other comparison logic...
       select foo;
}

public Foo getFoo(Foo target) {
    return FoosEqualTo(DC.foos, target).FirstOrDefault();
}