使用Linq向下钻取强类型对象

Question

我整个下午都在阅读有关Linq和Lambda表达式的教程。 我最终会得到它的，不会花我那么长时间，但是也许有人可以向我展示一个如何做我想做的事的示例，我会更快地掌握它。

我有一个客户端jQuery脚本，该脚本在我的Controller中调用一个返回JsonResult的例程。

给定ViewModel的：

public class Country
{
    [Key]
    public int Id { get; set; }

    [Index("UX_Country_CountryCode", IsUnique = true)]
    [MaxLength(5)]
    public string CountryCode { get; set; }

    public string CountryName { get; set; }

    public virtual ICollection<State> States { get; set; }
}

public class State
{
    [Key]
    public int Id { get; set; }

    [Index("UX_State_StateCodeCountryId", IsUnique = true, Order = 1)]
    [MaxLength(5)]
    public string StateCode { get; set; }

    public string StateName { get; set; }

    [ForeignKey("Country")]
    [Index("UX_State_StateCodeCountryId", IsUnique = true, Order = 0)]
    public int CountryId { get; set; }

    public virtual Country Country { get; set; }
}

示例数据：

new Country { Id = 1, CountryCode = "USA", CountryName = "United States" };
new Country { Id = 2, CountryCode = "CAN", CountryName = "Canada" };

new State { Id = 1, StateCode = "FL", StateName = "Florida", CountryId = 1 };
new State { Id = 2, StateCode = "CA", StateName = "California", CountryId = 1 };
new State { Id = 3, StateCode = "IA", StateName = "Iowa", CountryId = 1 };

使用/ Controller / StateList / USA的URI调用以下内容

[AllowAnonymous]
[AcceptVerbs(HttpVerbs.Get)]
public JsonResult StateList(string Id)
{
    // I need to know the Id of CountryCode == "USA"
    // Get <Country> with CountryCode == "USA". (I think this works)
    IEnumerable<SelectListItem> country = from c in GetCountryList()
                   where c.Value == Id
                   select c;

    // Now get all states where <State>.CountryId == country.Id
    var state = from State s in dbLocations.States
                   where s.CountryId == country.id
                   select s;

    return Json(new SelectList(state.ToArray(), "StateCode", "StateName"), JsonRequestBehavior.AllowGet);
}


public IEnumerable<SelectListItem> GetCountryList()
{
    var countries = new SelectList(dbLocations.Countries, "CountryCode", "CountryName").ToList();
    countries.Insert(0, (new SelectListItem { Text = "Select Country", Value = "-1" }));
    countries.Insert(1, (new SelectListItem { Text = "United Sates", Value = "USA" }));
    countries.Insert(2, (new SelectListItem { Text = "Canada", Value = "CAN" }));
    countries.Insert(3, (new SelectListItem { Text = "Mexico", Value = "MEX" }));
    countries.Insert(4, (new SelectListItem { Text = "Brazil", Value = "BRA" }));
    countries.Insert(5, (new SelectListItem { Text = "------------------------", Value = "-1" }));

    IEnumerable<SelectListItem> countrylist =
        countries.Select(m => new SelectListItem()
        {
           Text = m.Text,
           Value = m.Value
        });

    return countrylist;
}

我已经对StateList（）代码进行了几次尝试，只是不了解如何使用Linq做到这一点。 我认为GetCountryList（）可以-我在应用程序的其他位置多次使用它。 StateList（）最好如何编码？

另外，我找到了一些Linq教程，但没有点击。 有人知道一个好人吗？

Answer 1

最简单的方法是访问dbLocations并直接从数据库中获取国家。

[AllowAnonymous]
[AcceptVerbs(HttpVerbs.Get)]
public JsonResult StateList(string Id)
{
    // Get the country by Country Code
    var country = dbLocations.Countries.FirstOrDefault(c => c.CountryCode == Id);

    // Get all states where countryId equals the Id from the action
    var states = from State s in dbLocations.States
               where s.CountryId == country.Id
               select s;

    return Json(new SelectList(states.ToArray(), "StateCode", "StateName"), JsonRequestBehavior.AllowGet);
}

编辑：您可能会考虑将SelectedListItem的Value切换到Country.Id而不是Country.CountryCode ，无论如何在UI中看不到该值。 这样，通过其Id而不是CountryCode访问国家/地区会更容易