LINQ 有条件地添加连接

Question

我有一个 LINQ 查询，我试图从 2 个表中返回数据，但我加入的表是有条件的。

这是我想做的：

if (teamType == "A"){
    var query = from foo in context.People
                join foo2 in context.PeopleExtendedInfoA
                select foo;
}
else {
    var query = from foo in context.People
                join foo2 in context.PeopleExtendedInfoB
                select foo;
}

稍后我将进一步过滤查询。我显然无法以这种方式设置它，因为我将无法访问 if 块之外的“查询”，但它显示了我正在尝试做的事情。这是我稍后尝试对查询执行的操作的示例：

if (state != null)
{
     query = query.Where(p => p.State == state);
}

if (query != null) {
   var queryFinal = from foo in query
         select new PeopleGrid()
         {
              Name = foo.Name,
              Address = foo.Address,
              Hobby = foo2.Hobby
         }
}

我要返回的是表 foo 中的所有数据，然后是连接表中的一个字段，但根据逻辑，连接表会有所不同。 PeopleExtendedInfoA 和 PeopleExtendedInfoB 都具有列“爱好”，但我无法从连接表中访问“爱好”，这是我需要从连接表中获取的唯一字段。

我该怎么做呢？

Answer 1

PeopleExtendedInfoA 和 PeopleExtendedInfoB 是否继承自同一个基类？您可以创建一个 IQueryable<BaseClass> 并让 linq 提供程序在您添加连接时为您解决它。示例：

IQueryable<BasePeople> basePeople;
if (teamType == "A")
   basePeople = context.PeopleExtendedInfoA;
else
   basePeople = context.PeopleExtendedInfoB;

var query = from foo in context.People
            join foo2 in basePeople on foo.Id equals foo2.PeopleId
            select new PeopleGrid()
            {
              Name = foo.Name,
              Address = foo.Address,
              Hobby = foo2.Hobby
            };

Answer 2

试试这样：

 var queryFinal = from foo in query
                  where foo.State == state !=null ? state : foo.State
         select new PeopleGrid()
         {
              Name = foo.Name,
              Address = foo.Address,
              Hobby = foo2.Hobby
         }

Answer 3

您可以查询包含相关字段的中间类型，或者如果查询足够简单，您可以使用匿名类型，如下所示。重要的部分是 Join 调用都具有相同的返回类型（{ p.Name, a.Hobby } 和 { p.Name, b.Hobby } 都将是相同的匿名类型）。

var baseQuery = context.People;

var joined = type == "A" ?
    baseQuery.Join(PeopleExtendedInfoA,
        p => p.Id,
        a => a.PeopleId,
        (p, a) => new { p, a.Hobby }) :
    baseQuery.Join(PeopleExtendedInfoB,
        p => p.Id,
        b => b.PeopleId,
        (p, b) => new { p, b.Hobby });

var result = joined.Select(x => new
    {
        x.p.Name,
        x.p.Address,
        // etc.
        x.Hobby
    };

Answer 4

我想通了。感谢大家的所有回复，它让我的大脑再次工作并给了我一些新想法（即使我没有直接接受任何一个）我意识到我需要在最后而不是开始时那样做加入我不必处理不同类型的过滤。这就是我所做的：

var query = from foo in context.People
            select foo;

if (state != null)
{
    query = query.Where(p => p.State == state);
}

if (query != null) {
   if (teamType == "A")
   {
      var queryFinal = from foo in query
                       join foo2 in context.PeopleExtendedInfoA
      select new PeopleGrid()
      {
          Name = foo.Name,
          Address = foo.Address,
          Hobby = foo2.Hobby
      }
   }
   else
   {
      var queryFinal = from foo in query
                       join foo2 in context.PeopleExtendedInfoB
      select new PeopleGrid()
      {
          Name = foo.Name,
          Address = foo.Address,
          Hobby = foo2.Hobby
      }
   }
}