linq：在不同类型的条件下左加入 linq

Question

我有两张表，例如 PromotionalOffers 和 PromotionalOffersUsed
我正在使用连接获取匹配记录，现在我想包含不同的条件，例如 PromotionalOffers.ISVCSPId =10 和 PromotionalOffersUsed.OfferId 为 null

我已经使用 left join 在 sql 中编写查询，但我无法在 linq 中编写相同的查询

这里是我的 sql 查询

SELECT * 
FROM PromotionalOffers
left JOIN PromotionalOffersUsed ON PromotionalOffers.Id = PromotionalOffersUsed.OfferId where PromotionalOffers.ISVCSPId =10 and PromotionalOffersUsed.OfferId is null

我的 linq 查询是

(from s in db.PromotionalOffers
 join e in db.PromotionalOffersUsed on s.Id equals e.OfferId
 where s.ISVCSPId == iSVCSPData.Id
 select s).ToListAsync();

我无法在此处包含左连接

Answer 1

对于左连接，你必须这样做，以下是 linq 中左外连接的示例

var leftFinal =
        from l in lefts
        join r in rights on l equals r.Left into lrs
        from lr in lrs.DefaultIfEmpty()
        select new { LeftId = l.Id, RightId = ((l.Key==r.Key) ? r.Id : 0 };

查询

SELECT * 
FROM PromotionalOffers
left JOIN PromotionalOffersUsed ON PromotionalOffers.Id = PromotionalOffersUsed.OfferId where PromotionalOffers.ISVCSPId =10 and PromotionalOffersUsed.OfferId is null

linq 中的试探性查询

var leftFinal =
        (from l in PromotionalOffers.Where(p=> p.ISVCSPId ==10) 
        join r in PromotionalOffersUsed on l.ID equals r.OfferId  into lrs
        from lr in lrs.DefaultIfEmpty()
        select 
          new { LeftId = l.Id, RightId = ((l.ID==r.OfferId  ) ? r.OfferId   : -1 }
        ).where(d=> d.RightID != -1);

Answer 2

这样试试；

(from s in db.PromotionalOffers
 join e in db.PromotionalOffersUsed on s.Id equals e.OfferId into joinT
 from e in joinT.DefaultIfEmpty()
 where s.ISVCSPId == 10 && (e == null || e.OfferId == null)
 select new { PromotionalOffers = s, PromotionalOffersUsed = joinT } ).ToListAsync();

Answer 3

显然，PromotionalOffers 之间存在一对多关系 和PromotionalOffersUsed：每个PromotionalOffer 有零个或多个PromotionalOffersUsed，每个PromotionalOffersUsed 恰好属于一个PromotionalOffer。

如果您遵循entity framework conventions to design this one-to-many 关系，则不需要进行连接，也不需要左外连接，而是使用 ICollection 类：

class PromotionalOffer
{
    public int Id {set; set;}
    // a PromotionalOffer has zero or more PromotionalOfferUsed
    public virtual ICollection<PromotionalOfferUsed> PromitionalOffersUsed {get; set;}
    ...
}
class PromotionalOfferUsed
{
    public int Id {set; set;}
    // every PromotionalOffersUsed belongs to exactly one PromotionalOffer
    // using foreign key:
    public int PromotionalOfferId {get; set;}
    public PromotionalOffer PromotionalOffer {get; set;}
    ...
}
class MyDbContext: DbContext
{
    public DbSet<PromotionalOffer> PromotionalOffers {get; set;}
    public DbSet<PromotionalOfferUsed> PromotionalOffersUsed {get; set;
}

通常，这足以让实体框架知道您设计了一对多关系。

我们需要一些流畅的 API 的唯一原因是奇怪的复数：

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    // PromotionalOffersUsed are stored in a table with name PromotionOffersUsed:
    modelBuilder.Entity<PromotionalOfferUsed>().ToTable("PromotionalOffersUdsed");

    // Every PromotionalOffer has zero or more PromotionalOfferUsed
    // every PromotionalOfferUsed belongs to exactly one (required) PromotionalOffer
    // using foreign key PromtionalOfferId
    modelBuilder.Entity<PromotionalOffer>
        .HasMany(promotionalOffer => promotionalOffer.PromotionalOffersUsed)
        .WithRequired(promotionalOfferUsed => promotionOfferUsed.PromotionalOffer)
         .HasForeignKey(post => post.BlogId);

    base.OnModelCreating(modelBuilder);
}

再一次，如果您有标准的单数和复数（Person/Persons；Account/Accounts，Order/Orders），则不需要这个流畅的 API。

现在您的查询：考虑集合，而不是联接。

给我所有 ISVCSPId 等于 10 的 PromotionalOffers 他们所有的 PromotionalOffersUsed 的 OfferId 等于 null

using (var dbContext = new MyDbContext())
{
    var result = dbContext.PromotionalOffers
        .Where(promotionalOffer => promotionalOffer.ISVCSPID == 10)
        .Select(promotionalOffer => new
        {   // select only the properties you will be using:
            ISVCSPID = promotionalOffers.ISVSPID,
            ... // other properties from promotionalOffers

            PromotionalOffersUsed = promotionalOffers.PromtionalOffersUsed
                .Where(promotionalOffersUsed => promotionalOffersUsed.OfferId == null)
                .Select(promotionalOffersUsed => new
                {
                     // again: select only the properties of PromotionalOffersUsed
                     // that you will be using
                     Id = promotionalOffersUsed.Id,
                     ... // other properties
                })
                .ToList();
        }
}

因为实体框架知道有一个外键的一对多，它会为你做正确的左外连接。