C# 实体框架-包含的导航属性的选择条件

假设我有这些简化的EF生成实体…

public class PurchaseOrder
{
     public int POID {get;set;}
     public int OrderID {get;set;}
     public int VendorID {get;set;}
     public IEnumerable<Order> Orders {get;set;}
}

public class Order
{
     public int OrderID {get;set;}
     public decimal Price {get;set;}
     public IEnumerable<Item> Items {get;set;}
}

public class Item
{
     public int OrderID {get; set;}
     public string SKU {get;set;}
     public int VendorID {get;set;}
     public Order Order {get;set;}
}

这就完成了它的工作，并收回相关的实体，但是，我只想包括其VendorID与PurchaseOrder实体的VendorID匹配的项目实体

对于传统的SQL，我只是将其包含在连接条件中，但EF在内部构建这些条件

---

我可以使用什么LINQ magic来告诉EF应用条件，而不必手动创建实体之间的连接？

您不能有选择地拉回与特定条件匹配的某些子实体。你能做的最好的事情就是自己手动过滤掉相关的订单

public class PurchaseOrder
{
     public int POID {get;set;}
     public int OrderID {get;set;}
     public int VendorID {get;set;}
     public IEnumerable<Order> Orders {get;set;}

     public IEnumerable<Order> MatchingOrders {
         get {
            return this.Orders.Where(o => o.VendorId == this.VendorId);
         }
     }
}

公共类采购订单
{
公共int点{get；set；}
公共int-OrderID{get；set；}
public int VendorID{get；set；}
公共IEnumerable命令{get；set；}
公共IEnumerable匹配顺序{
得到{
返回this.Orders.Where（o=>o.VendorId==this.VendorId）；
}
}
}

你不能。EF不允许紧急加载的条件。您必须使用多个查询，如：

var pos = from p in context.PurchaseOrders.Include("Order")
          where ...
          select p;
var items = from i in context.Items
            join o in context.Orders on new { i.OrderId, i.VendorId} 
               equals new { o.OrderId, o.PurchaseOrder.VendorId }
            where // same condition for PurchaseOrders
            select i;

也可以在单个查询中使用投影：

var data = from o in context.Orders
           where ...
           select new
              {
                  Order = o,
                  PurchaseOrder = o.PurchaseOrder,
                  Items = o.Items.Where(i => i.VendorId == o.PurchaseOrder.VendorId)
              };

---

您可以在此处使用IQueryable扩展：

扩展动态构建匿名类型。这将用于@Ladislav Mrnka所述的投影

然后你可以这样做：

var query = query.SelectIncluding( new List<Expression<Func<T,object>>>>(){

//Example how to retrieve only the newest history entry
x => x.HistoryEntries.OrderByDescending(x => x.Timestamp).Take(1),

//Example how to order related entities
x => x.OtherEntities.OrderBy(y => y.Something).ThenBy(y => y.SomeOtherThing),

//Example how to retrieve entities one level deeper
x => x.CollectionWithRelations.Select(x => x.EntityCollectionOnSecondLevel),

//Of course you can order or subquery the deeper level
//Here you should use SelectMany, to flatten the query
x => x.CollectionWithRelations.SelectMany(x => x.EntityCollectionOnSecondLevel.OrderBy(y => y.Something).ThenBy(y => y.SomeOtherThing)),

});

var query=query.selectinclude（新列表>（）{
//示例如何仅检索最新的历史记录条目
x=>x.HistoryEntries.OrderByDescending（x=>x.Timestamp）.Take（1），
//如何对相关实体进行排序的示例
x=>x.OtherEntities.OrderBy（y=>y.Something）。然后by（y=>y.Something），
//示例如何检索更深一层的实体
x=>x.CollectionWithRelations.Select（x=>x.EntityCollectionsSecondLevel），
//当然，您可以对更深层次进行排序或子查询
//在这里，您应该使用SelectMany来展开查询
x=>x.CollectionWithRelations.SelectMany（x=>x.EntityCollectionsSecondLevel.OrderBy（y=>y.Something）。然后通过（y=>y.Something）），
});

我希望在未来将此视为一项功能增强。五年多了，但仍然一无所获。。。令人惊讶的是，对于EF可能拥有的最有用的功能之一来说，这并不坏

var query = query.SelectIncluding( new List<Expression<Func<T,object>>>>(){

//Example how to retrieve only the newest history entry
x => x.HistoryEntries.OrderByDescending(x => x.Timestamp).Take(1),

//Example how to order related entities
x => x.OtherEntities.OrderBy(y => y.Something).ThenBy(y => y.SomeOtherThing),

//Example how to retrieve entities one level deeper
x => x.CollectionWithRelations.Select(x => x.EntityCollectionOnSecondLevel),

//Of course you can order or subquery the deeper level
//Here you should use SelectMany, to flatten the query
x => x.CollectionWithRelations.SelectMany(x => x.EntityCollectionOnSecondLevel.OrderBy(y => y.Something).ThenBy(y => y.SomeOtherThing)),

});