Question

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;}
}

业务逻辑：

订单可以包含多个PO，订单上的每个不同供应商都有一个（供应商在商品级别确定）。

如何选择性地包含子实体？

在查询PO时，我想自动为订单和项目添加子项目。

我使用Include（）...

完成此任务

Context.PurchaseOrders.Include("Orders.Items");

这是工作并撤回相关实体，但是， 我只想包含其VendorID与PurchaseOrder实体的VendorID匹配的Item实体 。

对于传统的SQL，我只是将它包含在JOIN条件中，但EF在内部构建它们。

我可以使用哪种LINQ魔法告诉EF应用条件，而无需在实体之间手动创建JOIN？

Answer 1

您无法有选择地撤回符合特定条件的某些子实体。您可以做的最好的事情是亲自手动过滤相关订单。

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);
         }
     }
}

Answer 2

你做不到。 EF不允许急切加载的条件。您必须使用多个查询，例如：

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;

或者您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)
              };

Answer 3

您可以在此处使用IQueryable-Extensions：

https://github.com/thiscode/DynamicSelectExtensions

Extension以动态方式构建匿名类型。这将用于@ Ladislav-Mrnka所描述的投影。

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)),

});

实体框架 - 包含导航属性的选择条件

3 个答案: