[英]Entity Framework lazy loading in using statement
我很好奇,当我深入到实体框架时,很高兴在下面显示的这个示例中帮助我了解延迟加载:
public partial class Customer
{
public int CustomerID { get; set; }
public Nullable<int> PersonID { get; set; }
public Nullable<int> StoreID { get; set; }
public Nullable<int> TerritoryID { get; set; }
public string AccountNumber { get; set; }
public System.DateTime ModifiedDate { get; set; }
public virtual Territory Territory { get; set; }
}
public partial class Territory
{
public Territory()
{
this.Customers = new HashSet<Customer>();
}
public int TerritoryID { get; set; }
public string Name { get; set; }
public string CountryRegionCode { get; set; }
public string C_Group_ { get; set; }
public decimal SalesYTD { get; set; }
public decimal SalesLastYear { get; set; }
public virtual ICollection<Customer> Customers { get; set; }
}
public ActionResult LoadData()
{
var draw = Request.Form.GetValues("draw").FirstOrDefault();
var start = Request.Form.GetValues("start").FirstOrDefault();
var length = Request.Form.GetValues("length").FirstOrDefault();
var sortColumn = Request.Form.GetValues("columns[" + Request.Form.GetValues("order[0][column]").FirstOrDefault() + "][name]").FirstOrDefault();
var sortColumnDir = Request.Form.GetValues("order[0][dir]").FirstOrDefault();
var pageSize = length != null ? Convert.ToInt32(length) : 0;
var skip = start != null ? Convert.ToInt32(start) : 0;
int totalRecords = 0;
using (MyDatabaseEntities dc = new MyDatabaseEntities())
{
//1.
var items = dc.Customers.Select(a => a);
//2.
//var items = dc.Customers.Select(a => new
//{
// a.CustomerID,
// a.PersonID,
// a.StoreID,
// TerritoryName = a.Territory.Name,
// a.AccountNumber,
// a.ModifiedDate
//});
if (!(string.IsNullOrEmpty(sortColumn) && string.IsNullOrEmpty(sortColumnDir)))
{
items = items.OrderBy(sortColumn + " " + sortColumnDir);
}
totalRecords = items.Count();
var data = items.Skip(skip).Take(pageSize).ToList();
return Json(new { recordsFiltered = totalRecords, recordsToral = totalRecords, data = data }, JsonRequestBehavior.AllowGet);
}
}
我已经编号了一些部分。 在您看到的第一种情况下,我选择了所有客户,延迟加载为true,这会引发错误:
ObjectContext实例已被处置,不能再用于需要连接的操作
如果我取消注释第二部分,它将正常工作。 它们之间的唯一区别是在第二种情况下,我选择了我需要的列
这是因为JsonResult
(来自return Json(...)
语句)将由ASP在离开using
块后执行,并且它将尝试序列化Customer
对象的所有字段,并在dbcontext具有后尝试到达Territory
引用属性。被处置。
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