我認爲那些連接沒有被正確鏈接。
在第二次連接中，第二個參數引用了外部集合。這將是第一次加入new { User = user, SchemeName = schemeName })的預計匿名類型。
假設有對用戶類的屬性LifeAssurance，然後我猜你想要做的第二加入基於從用戶對象屬性，像這樣：
var trs = db.Users
.Join(db.SchemeNames,
user => user.Pension.PensionSchemeNameID,
schemeName => schemeName.SchemeNameID,
(user, schemeName) => new { User = user,
SchemeName = schemeName })
.Join(db.SchemeNames,
x => x.User.LifeAssurance.LifeAssuranceSchemeNameID,
schemeName => schemeName.SchemeNameID,
(x, schemeName) => new { User = x.User,
PensionSchemeName = x.SchemeName,
LifeAssuranceSchemeName = schemeName })
.Where(a => UserIDs.Contains(a.User.UserName))
.Select(a => new TRSViewModel{
UserName = a.User.UserName,
FirstName = a.User.UserDetails.FirstName,
LastName = a.User.UserDetails.LastName,
Salary = a.User.UserDetails.Salary,
PensionSchemeName = a.PensionSchemeName.Name,
LifeAssuranceSchemeName = a.LifeAssuranceSchemeName.Name
});
這可以寫上查詢語法（可能更易於閱讀）：
var trs = from user in db.Users
join pensionSchema in db.SchemeNames
on user.Pension.PensionSchemeNameID equals pensionSchema.SchemeNameID
join lifeAssuranceSchema in db.SchemeNames
on user.LifeAssurance.LifeAssuranceSchemeNameID equals lifeAssuranceSchema.SchemeNameID
where UserIds.Contains(user.UserName)
select new TRSViewModel{
UserName = user.UserName,
FirstName = user.UserDetails.FirstName,
LastName = user.UserDetails.LastName,
Salary = user.UserDetails.Salary,
PensionSchemeName = pensionSchema.Name,
LifeAssuranceSchemeName = lifeAssuranceSchema.Name }
最後，考慮到這些都是內部連接，因此任何用戶沒有退休金或人壽保險架構將不予退還。如果你想左連接，你可以不喜歡的方式描述here（雖然我猜你不需要這個否則像user.Pension.PensionSchemeNameID表達式將拋出一個NullReferenceException如果有用戶沒有退休金）
var query =
from user in db.Users
from pensionScheme in db.SchemeNames
.Where(s => s.SchemeNameID == user.Pension.PensionSchemeNameID)
.DefaultIfEmpty()
from lifeAssuranceScheme in db.SchemeNames
.Where(s => s.SchemeNameID == user.LifeAssurance.LifeAssuranceSchemeNameID)
.DefaultIfEmpty()
select new { User = user,
Pension = pensionScheme,
LifeAssurance = lifeAssuranceScheme}
我也很快測試在內存中的集合使用下面的代碼與查詢，但他們應該通過實體框架被正確地轉換爲SQL：
public class SchemeName
{
public int SchemeNameID { get; set; }
public string Name { get; set; }
}
public class Pension
{
public int PensionSchemeNameID { get; set; }
}
public class LifeAssurance
{
public int LifeAssuranceSchemeNameID { get; set; }
}
public class User
{
public string Name { get; set; }
public Pension Pension { get; set; }
public LifeAssurance LifeAssurance { get; set; }
}
public static class db
{
public static IEnumerable<User> Users = new List<User>() {
new User { Name = "User 1",
Pension = new Pension { PensionSchemeNameID = 1 },
LifeAssurance = new LifeAssurance { LifeAssuranceSchemeNameID = 2 } },
new User { Name = "User 2",
Pension = new Pension { PensionSchemeNameID = 1 },
LifeAssurance = new LifeAssurance { LifeAssuranceSchemeNameID = 2 } },
new User { Name = "User 3",
Pension = new Pension { PensionSchemeNameID = 1 },
LifeAssurance = new LifeAssurance { LifeAssuranceSchemeNameID = -999 }},
new User { Name = "User 4",
Pension = new Pension { PensionSchemeNameID = -999 },
LifeAssurance = new LifeAssurance { LifeAssuranceSchemeNameID = 2 } }
};
public static IEnumerable<SchemeName> SchemeNames = new List<SchemeName>() {
new SchemeName{ SchemeNameID = 1, Name = "Scheme 1" },
new SchemeName{ SchemeNameID = 2, Name = "Scheme 2" }
};
}
private void Run()
{
var innerJoinQuery1 = db.Users
.Join(db.SchemeNames,
user => user.Pension.PensionSchemeNameID,
schemeName => schemeName.SchemeNameID,
(user, schemeName) => new
{
User = user,
SchemeName = schemeName
})
.Join(db.SchemeNames,
x => x.User.LifeAssurance.LifeAssuranceSchemeNameID,
schemeName => schemeName.SchemeNameID,
(x, schemeName) => new
{
User = x.User,
PensionSchemeName = x.SchemeName,
LifeAssuranceSchemeName = schemeName
})
.Where(a => a.User.Name.StartsWith("User "))
.Select(a => new
{
UserName = a.User.Name,
PensionSchemeName = a.PensionSchemeName.Name,
LifeAssuranceSchemeName = a.LifeAssuranceSchemeName.Name
});
var innerJoinQuery2 = from user in db.Users
join pensionSchema in db.SchemeNames
on user.Pension.PensionSchemeNameID equals pensionSchema.SchemeNameID
join lifeAssuranceSchema in db.SchemeNames
on user.LifeAssurance.LifeAssuranceSchemeNameID equals lifeAssuranceSchema.SchemeNameID
where user.Name.StartsWith("User ")
select new
{
UserName = user.Name,
PensionSchemeName = pensionSchema.Name,
LifeAssuranceSchemeName = lifeAssuranceSchema.Name
};
var lefJoinQuery =
from user in db.Users
from pensionScheme in db.SchemeNames
.Where(s => s.SchemeNameID == user.Pension.PensionSchemeNameID)
.DefaultIfEmpty()
from lifeAssuranceScheme in db.SchemeNames
.Where(s => s.SchemeNameID == user.LifeAssurance.LifeAssuranceSchemeNameID)
.DefaultIfEmpty()
select new
{
UserName = user.Name,
PensionSchemeName = pensionScheme != null ? pensionScheme.Name : "No Pension",
LifeAssuranceSchemeName = lifeAssuranceScheme != null ? lifeAssuranceScheme.Name : "No Life Assurance"
};
foreach (var result in innerJoinQuery1)
Print(result.UserName, result.PensionSchemeName, result.LifeAssuranceSchemeName);
Console.WriteLine();
foreach (var result in innerJoinQuery2)
Print(result.UserName, result.PensionSchemeName, result.LifeAssuranceSchemeName);
Console.WriteLine();
foreach (var result in lefJoinQuery)
Print(result.UserName, result.PensionSchemeName, result.LifeAssuranceSchemeName);
Console.WriteLine();
Console.ReadKey();
}
private void Print(string user, string pension, string lifeAssurance)
{
Console.WriteLine(String.Format("User: '{0}', Pension: '{1}', Life Assurance: '{2}'", user, pension, lifeAssurance));
}
}
將會產生預期的輸出：
對於第一內連接查詢
用戶： '用戶1'，退休金： '方案1'，人壽保險： '方案2'
用戶： '用戶2'，年金：'方案1' ，人壽保險： '方案2'
對於第二內連接查詢
用戶： '用戶1'，退休金： '方案1'，人壽保險： '方案2'
用戶： '用戶2'，養老金： '方案1'，人壽保險： '計劃2'
左連接查詢
用戶： '用戶1'，養老金： '方案1'，人壽保險： '方案2'
用戶： '用戶2'，養老金： '方案1'，人壽保險： '方案2'
用戶： '用戶3'，養老金：'計劃1'，人生保證：'沒有生命保證」
用戶： '用戶4'，養老： '沒有退休金'，人壽保險： '計劃2'
都在同一個表中的'PensionSchemeName'和'LifeAssuranceSchemeName'值？ –