Entity Framework 中的左连接具有 1 对多关系

Question

我有五个表：

Coupons
Coupon_Redemptions
User_Coupon
Wash
Account

这是在SQL Server和Entity Raw SQL中工作的SQL Query ，但无法使其与LINQ因为1-to-many关系，延迟加载无法获得它，因为 User_coupons、Wash...集合。

SELECT 
    C.Id AS "CouponId", C.Coupon_code AS "CouponCode", 
    C.Discount_amount AS "DiscountAmount", 
    C.Valid_for_all AS "ValidForAll", C.Expiration_date AS "ExpirationDate", 
    R.Redemption_date AS "RedemptionDate",
    U.Date_added AS "DateAddedToUser", W.Id AS "WashId", A.Name  
FROM 
    Coupon C
    LEFT JOIN User_coupon U on U.CouponId = C.Id
    LEFT JOIN Coupon_redemption R on R.CouponId = C.Id
    LEFT JOIN Wash W on W.CouponId = C.Id
    LEFT JOIN Account A on U.AccountId = A.Id

这是关系图的片段

尝试对此查询进行变体，它返回一行。 但它看起来很完美。

var results =
                from c in db.Coupons
                from u in c.User_coupon.DefaultIfEmpty()
                from r in c.Coupon_redemption.DefaultIfEmpty()
                from w in c.Washes.DefaultIfEmpty()
                select new {
                    CouponId = c.Id,
                    CouponCode = c.Coupon_code,
                    DiscountAmount = c.Discount_amount,
                    ValidForAll = c.Valid_for_all,
                    ExpirationDate = c.Expiration_date,
                    RedemptionDate = r.Redemption_date,
                    DateAddedToUser = u.Date_added,
                    WashId = w.Id
                };

结果到字符串：

SELECT[Extent1].[Id] AS [Id], [Extent1].[Coupon_code] AS [Coupon_code], [Extent1].[Discount_amount] AS [Discount_amount], [Extent1].[Valid_for_all] AS [Valid_for_all], [Extent1].[Expiration_date] AS [Expiration_date], [Extent3].[Redemption_date] AS [Redemption_date], [Extent2].[Date_added] AS [Date_added], [Extent4].[Id] AS [Id1] 
FROM [dbo].[Coupon] AS [Extent1] 
LEFT OUTER JOIN [dbo].[User_coupon] AS [Extent2] ON [Extent1].[Id] = [Extent2].[CouponId]
LEFT OUTER JOIN [dbo].[Coupon_redemption] AS [Extent3] ON [Extent1].[Id] = [Extent3].[CouponId]
LEFT OUTER JOIN [dbo].[Wash] AS [Extent4] ON [Extent1].[Id] = [Extent4].[CouponId]

Answer 1

好吧，当包含它们的对象被具体化（即您通过对象实例工作）时，它们是集合（有或没有延迟加载）。 在 LINQ to Entities 查询中使用时，它们是简单的表导航（连接）。

规则很简单。 要获得集合导航属性的等效 SQL 内连接，您可以使用

from child in parent.Collection

和分别对于左外连接：

from child in parent.Collection.DefaultIfEmpty()

对于参考导航属性，您不能明确指定连接的类型 - 它取决于（受控制）关系是必需的还是可选的。 而不是from ，您可以使用let或直接使用导航属性来获取等效的 SQL 查询连接。

话虽如此，等效的 LINQ 查询将是这样的：

var query = 
    from c in dbContext.Coupons
    from u in c.User_coupon.DefaultIfEmpty()
    from r in c.Coupon_redemptions.DefaultIfEmpty()
    from w in c.Washes.DefaultIfEmpty()
    let a = u.Account
    select new
    {
        CouponId = c.Id,
        CouponCode = c.Coupon_code,
        DiscountAmount = c.Discount_amount,
        ValidForAll = c.Valid_for_all,
        ExpirationDate = c.Expiration_date, 
        RedemptionDate = r.Redemption_date,
        DateAddedToUser = u.Date_added,
        WashId = w.Id,
        Name = a.Name,          
    };