[英]joining two tables in entity framework
我正在嘗試在實體框架中加入兩個表,並從其中一個表中獲取值,以對第三個表進行另一個查詢,這就是我正在使用的查詢
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 };
但是它給我帶來了語法錯誤。join子句中的表達式之一的類型不正確。 在對“ GroupJoin”的調用中類型推斷失敗,我已經搜索了該錯誤,發現人們總是說要確保equals子句中的屬性是同一類型,是的,所有類型都是非可空int
進行LINQ連接時,equals兩側的類型必須完全相同,但是在查詢中您具有USER_ID和userId。
解決方法很簡單:
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的流利語法(為什么要在這里實際執行操作,由於使用了“ into”子句;常規Join與此類似),那么為什么需要這樣做很容易。
簽名是:
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
)
請注意,outerKeySelector和innerKeySelector必須返回相同的TKey類型(然后通過匹配這些鍵來完成連接)。
要以流利的風格編寫原始聯接,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);
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()
嗎?因為您的模型將允許客戶/描述與多個帳戶相關)
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