我正在尝试在实体框架中连接两个表,并从其中一个表中获取值以在第三个表上执行另一个查询 这是我正在使用的查询
var fav = from favs in db.FAVORITES
join pins in db.PINS
on new { favs.USER_ID, favs.PIN_ID } equals new { userId, pins.PIN_ID } into res
from r in res
select new { favs.PIN_ID, r.TYPE_ID };
但它给我一个语法错误 join子句中某个表达式的类型不正确。调用“GroupJoin”时类型推断失败 我搜索了错误并发现人们总是说要确保 equals 子句中的属性是相同的类型,是的,所有类型都是非可空的int
答案 0 :(得分:3)
在进行LINQ连接时,equals两侧的类型必须完全相同,但在查询中,您有USER_ID与userId。
修复只是:
var fav = from favs in db.FAVORITES
join pins in db.PINS
on new { favs.USER_ID, favs.PIN_ID }
equals
// use explicit naming so the first property gets the name USER_ID not userId
new { USER_ID = userId, pins.PIN_ID }
into res
from r in res
select new { favs.PIN_ID, r.TYPE_ID };
如果使用GroupJoin的流畅语法(由于“into”子句,你实际上在这里做了什么,常见的Join类似),可以更容易理解为什么这是必要的。
签名是:
public static IQueryable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(
this IQueryable<TOuter> outer,
IEnumerable<TInner> inner,
Expression<Func<TOuter, TKey>> outerKeySelector,
Expression<Func<TInner, TKey>> innerKeySelector,
Expression<Func<TOuter, IEnumerable<TInner>, TResult>> resultSelector
)
请注意,outerKeySelector和innerKeySelector必须返回相同类型的TKey(然后通过匹配这些键来完成连接)。
要以流畅的方式编写原始联接,您需要:
var fav = db.FAVORITES.GroupJoin(
inner: inner,
// the return types of the selectors don't match, so the compiler can't
// infer a type for TKey!
outerKeySelector: favs => new { favs.USER_ID, favs.PIN_ID },
innerKeySelector: pins => new { userId, pins.PIN_ID },
resultSelector: (favs, res) => res.Select(r => new { favs.PIN_ID, r.TYPE_ID })
)
.SelectMany(res => res);
答案 1 :(得分:-3)
var balance = (from a in context.Accounts
join c in context.Clients on a.UserID equals c.UserID
where c.ClientID == yourDescriptionObject.ClientID
select a.Balance)
.SingleOrDefault();
var balance = (from a in context.Accounts
join c in context.Clients on a.UserID equals c.UserID
join d in context.Descriptions on c.ClientID equals d.ClientID
where d.DescriptionID == yourDescriptionID
select a.Balance)
.SingleOrDefault();
(或FirstOrDefault()
或ToList()
或Sum()
?因为您的模型允许客户/说明与多个帐户相关)