ASP.NET C# MVC 实体框架

Question

我正在学习 ASP.NET MVC，但我一直坚持使用实体框架创建复杂的 SQL - 它并不复杂，但它可能会变得复杂。

我的常规 ASP.NET C# 网络表单如下所示，我想将其转换为 ASP.NET MVC 并使用 Entity Framework。

string date = Request.QueryString["date"] ?? "";
string strWhere = "";
string sqlTop = "20";

DateTime temp;

if (DateTime.TryParse(date, out temp))
{
    strWhere = "WHERE CAST(NewsDate AS date)='" + temp.ToString("yyyy-MM-dd") + "'";
    sqlTop = "50";
}

using (SqlConnection con = new SqlConnection(WebConfigurationManager.ConnectionStrings["NewsDB"].ConnectionString))
{
    con.Open();
    string selectSql = @"SELECT TOP " + sqlTop + @" *
                              , (SELECT TOP 1 FileName 
                                 FROM NewsImage
                                 WHERE NewsGuid = N.NewsGuid
                                 ORDER BY FileOrder ASC) AS FileName
                        FROM    News N " + strWhere + @"
                        ORDER BY N.NewsDate DESC";

    using (SqlCommand selectCmd = new SqlCommand(selectSql, con))
    {
        using (SqlDataReader selectDr = selectCmd.ExecuteReader())
        {
            while (selectDr.Read())
            {
                //My Output logic goes here
            }
        }
    }
}

我的问题是如何使用实体框架实现这种类型的 SQL。

我已经使用数据库优先的方法创建了实体并设置了DbContext。

private NewsEntities db = new NewsEntities();

我有两个数据库表：News & NewsImage 单个新闻可以有多个新闻图像（1-N）

我尝试过实现这样的目标：

新闻类

public class NewsClass
    {
        public News NewsDetails { get; set; }
        public NewsImage NewsImages { get; set; }
    }

型号：新闻

public partial class News
{
    public int Id { get; set; }
    public System.Guid NewsGuid { get; set; }
    public string Heading { get; set; }
    public string Text { get; set; }
    public string Author { get; set; }
    public System.DateTime NewsDate { get; set; }
}

模型：NewsImage

public partial class NewsImage
{
    public int Id { get; set; }
    public System.Guid NewsGuid { get; set; }
    public string FileName { get; set; }
    public int FileOrder { get; set; }
}

控制器：

var newsList = from n in news
join img in newsImages on n.NewsGuid equals img.NewsGuid into table1
from img in table1.DefaultIfEmpty()
select new NewsClass { NewsDetails = n, NewsImage = img };

从这里我被困住了...... 子查询上的“select top 1”，如何实现这个

越来越好... 现在我做了这样的事情：

var newsImages = db.NewsImages.OrderBy(n => n.NewsGuid).OrderBy(n => n.FileOrder).Where(n => n.FileOrder == 10);
var allNews = db.News.OrderByDescending(n => n.NewsDate).Take(sqlTop);

if (validDate)
    allNews = allNews.Where(n => n.NewsDate.Year == newsDate.Year && n.NewsDate.Month == newsDate.Month && n.NewsDate.Day == newsDate.Day);

现在我需要将“newsImages”和“allNews”这两个对象合并到一个列表“newsList”中。

如何将查询 allNews 和 newsImages 的两个列表合并到一个列表中，如下所示：

select new NewsList
{
    NewsGuid = n.NewsGuid,
    Heading = n.Heading,
    FileName = ni.FileName
}).Take(sqlTop).ToList<NewsList>();

Answer 1

Entity Framework（和其他 ORM）的主要好处是您不必再编写原始 SQL 来实现您的目标。 OR 映射器创建了一个很好用的对象结构，您可以使用它来进行查询。

在你的情况下 - 你会做类似的事情：

string date = Request.QueryString["date"] ?? "";
int sqlTop = 20;

DateTime? newsDate = null;

if (DateTime.TryParse(date, out DateTime temp))
{
    newsDate = temp;
    sqlTop = 50;
}

using (NewsEntities db = new NewsEntities())
{
    // grab the "NewsClass" entries - include the "NewsDetails" and
    // "NewsImage" navigation property
    var query = db.NewsClass
                  .Include(n => n.NewsDetails);
                  .Include(n => n.NewsImage);

    // if you have a valid date - use that to refine the query
    if (newsDate.HasValue)
    {
        query = query.Where(n => n.NewsDetails.NewsDate == newsDate.Value);
    }

    // order by NewsDate, and select only "sqlTop" entries 
    query = query.OrderByDescending(n => n.NewsDetails.NewsDate
                 .Take(sqlTop);

    // now you can iterate over the "News" objects returned from the query
    foreach(News n in query)
    {
       // get the first image from navigation property
       var firstImage = n.NewsImage
                         .OrderBy(img => img.FileOrder)
                         .FirstOrDefault();

       // do whatever you want with your news - output what you need
    }
}