[英]Linq left join with Entity Framework
我有一個Vehicles表,我需要對VehicleAttribute進行左連接。
var vehicle = (from v in context.Vehicles
//join vehicleAttributes
join va in context.VehicleAttributes on v.VehicleId equals va.VehicleId into vAttributes
from vehicleAttributes in vAttributes.DefaultIfEmpty()
where v.VehicleId == vehicleId
select new { v, vehicleAttributes });
到現在為止還挺好。 VehicleAttribute還具有一列AttributeId。 我只需要在此列表中加入這些VehicleAttributes:
List<Guid> allowedAttributes = (from ua in context.UserAttributes
where ua.UserId == UserSession.CurrentUser.UserId
select ua.AttributeId).ToList();
我該怎么做? 我認為子查詢可能是正確的方法,但我正在努力。
感謝所有答案。
編輯:一種不同的方法來解釋我的問題:我有這兩個查詢
SELECT Vehicle.VehicleId,VehicleAttribute.AttributeId
FROM Vehicle
LEFT JOIN VehicleAttribute
ON Vehicle.VehicleId = VehicleAttribute.VehicleId
SELECT UserAttribute.AttributeId
FROM UserAttribute
WHERE UserAttribute.UserId = '4D0F8AD2-7A4D-4E29-A6D3-E5FCD6075388'
並希望將它們組合在一起,因此我只獲得第二個查詢中的屬性ID。 where子句不起作用,因為即使沒有attributeIds,我仍然想要vehicleId
定義allowedAttributes
您可以更改
vAttributes.DefaultIfEmpty()
您的第一個查詢:
vAttributes
.Where(va => allowedAttributes.Contains(va.AttributeId))
.DefaultIfEmpty()
不確定這是您的情況。 您可以嘗試使用子查詢向左聯接:
//define the query that returns allowed VehicleAttributes
allowedAttributesQuery =from ua in context.UserAttributes
where ua.UserId == UserSession.CurrentUser.UserId
select ua.VehicleAttribute; //I suppose that VehicleAttribute navigation property exists in the UserAttribute
//query that joins Vehicles with the allowedAttributes subquery
var vehicle = (from v in context.Vehicles
join va in allowedAttributesQuery on v.VehicleId equals va.VehicleId into vAttributes
from vehicleAttributes in vAttributes.DefaultIfEmpty()
where v.VehicleId == vehicleId
select new { v, vehicleAttributes });
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