C# Linq与实体框架的左连接

我有一个车辆表，我需要在VehicleAttribute上执行左联接

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 });

到目前为止还不错。VehicleAttribute还有一个列AttributeId。我只需要加入以下列表中的车辆属性：

List<Guid> allowedAttributes = (from ua in context.UserAttributes
                                                    where ua.UserId == UserSession.CurrentUser.UserId
                                                    select ua.AttributeId).ToList();

并希望将它们组合在一起，以便只获取第二个查询中的属性id。

---

where子句不起作用，因为即使没有AttributeID，我仍然需要vehicleId。不确定这是您的场景。您可以尝试使用子查询进行左联接：

//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 });

---

不确定这是你的情况。您可以尝试使用子查询进行左联接：

//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 });

---

定义

allowedAttributes

后，可以更改

vAttributes.DefaultIfEmpty()

您第一次查询的目标：

vAttributes
    .Where(va => allowedAttributes.Contains(va.AttributeId))
    .DefaultIfEmpty()

---

定义

allowedAttributes

后，可以更改

vAttributes.DefaultIfEmpty()

您第一次查询的目标：

vAttributes
    .Where(va => allowedAttributes.Contains(va.AttributeId))
    .DefaultIfEmpty()

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您更应该将导航属性与EF一起使用，而不是显式联接。我假设您的映射设置正确，因为这里没有显示它们……您的第一个代码段不完整，存在未定义的标识符。你能发布整个查询吗？当然我更新了我的帖子：-）你更应该使用带有EF的导航属性，而不是显式连接。我假设您的映射设置正确，因为这里没有显示它们……您的第一个代码段不完整，存在未定义的标识符。你能发布整个查询吗？当然，我更新了我的帖子：-）