Question

var topTen = (from ep in db.VisitedCities
                            join c in db.Countries on ep.PersonWhoVisitedNationalityId equals c.CountryId
                            join e in db.Cities on ep.CityId equals e.CityId
                            join t in db.Countries on e.CountryId equals t.CountryId
                            where t.Name == "Portugal"
                            select ep.PersonWhoVisitedNationality).ToList();

结果会返回一个包含多个项目的列表但是所有这些项目都是null，我在这里缺少什么？

我希望得到一份国籍清单（它们属于Country）

提前致谢。

编辑：

好的，所以第一个问题与此有关，我最终想要的是这样的（只有当我把.ToList()放在中间时才有效:(）

var topTen = (from ep in db.VisitedCities
                              join e in db.Cities on ep.CityId equals e.CityId
                              join t in db.Countries on e.CountryId equals t.CountryId
                              where t.Name == "Portugal"
                              select ep)**.ToList()**
                              .GroupBy(x => x.PersonWhoVisitedNationality)
                              .Select(cp => new
                              {
                                  CountryName = cp.Key,
                                  NumberOfTravelers = cp.Count()
                              })
                              .OrderByDescending( x => x.NumberOfTravelers)
                              .Take(10)
                              .ToList();

请注意，我正在使用新的实体框架7，我认为现在包含扩展功能还没有... 所以到了夏天，这个查询工作正常，但只有.ToList()位于中间:(

Answer 1

好的，因为EF7仍然处于测试阶段并且查询速度非常慢，我最终为EF7创建了一个扩展方法，以便进行原始SQL查询，最终结果如下：

var top10PT = db.Database.ExecuteSqlCommandExtensionMethod(@"
            select top 10 Nationality.Name, count(*) AS NumberOfTravelers FROM
            (
            select  [PersonWhoVisitedId]   FROM VisitedCity 
            INNER JOIN City ON VisitedCity.CityId = City.CityId 
            INNER JOIN Country ON City.CountryId = Country.CountryId
            WHERE Country.Alpha2Code = 'PT'
            ) AS Subquery
            INNER JOIN Person ON Person.PersonId = Subquery.PersonWhoVisitedId
            INNER JOIN Country as Nationality ON Person.NationalityId = Nationality.CountryId
            GROUP BY Nationality.Name
            ORDER BY NumberOfTravelers desc
            ").ToList();

现在它真的很快：D。

这是我的扩展方法，以防有人想知道：

public static class Entity7Extensions
{
    public static IEnumerable<dynamic> ExecuteSqlCommandExtensionMethod(this DatabaseFacade database, string sql)
    {
        var connection = database.GetDbConnection();
        var command = connection.CreateCommand();
        command.CommandText = sql;

        try
        {
            connection.Open();

            var reader = command.ExecuteReader();

            while (reader.Read())
            {
                var expandoObject = new ExpandoObject() as IDictionary<string, object>;

                for (var i = 0; i < reader.FieldCount; i++)
                {
                    object propertyValue = reader.GetValue(i);

                    if (propertyValue is System.DBNull)
                        propertyValue = null;

                    expandoObject.Add(reader.GetName(i), propertyValue);
                }

                yield return expandoObject;
            }
        }
        finally
        {
            connection.Close();
        }
    }
}

如何获取此linq查询以返回正确的值

1 个答案: