[英]How to get this linq query to return correct values
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();
這樣的結果返回一個包含多個項目的列表,但是所有項目都為空,我在這里錯過了什么?
我希望得到一份國籍列表(它們是Country
類型的)
提前致謝。
編輯:
好的,所以第一個問題與此相關,我最終想要的是這樣的東西(它可以工作,但.ToList()
是我將.ToList()
放在中間:()
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();
請注意,我正在使用新的實體框架7,並且我認為現在include擴展尚不起作用...因此,在夏季,此查詢可以正常工作,但.ToList()
是.ToList()
位於中間:(
好的,因為EF7仍處於測試階段,並且查詢速度非常慢,所以我最終為EF7創建了擴展方法,以便執行原始SQL查詢,並最終如下所示:
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();
現在真的非常快:D。
如果有人想知道,這是我的擴展方法:
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();
}
}
}
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