我如何将 "or" 运算符引入 linq 查询连接

Question

将 "or" 运算符引入 linq 查询连接的语法是什么。好像不喜欢“||”，"or"？我需要将一个外键与来自用户 table.

的两个可能的键相结合

var thirdQuery = (from u in tbl_users
join ua in tbl_userDocuments on (u.TransportUserID equals ua.fkDocumentID
                             || u.WorkUserID equals ua.fkDocumentID) into uaGroup
from uaTrans in uaGroup.DefaultIfEmpty()
join ul in tbl_user_logins on uaTrans.fkUserID equals ul.user_login_id_pk into ulGroup

没有 or 部分，这有效但缺少额外的键

var thirdQuery = (from u in tbl_users
join ua in tbl_userDocuments on (u.TransportUserID equals ua.fkDocumentID) into uaGroup
from uaTrans in uaGroup.DefaultIfEmpty()
join ul in tbl_user_logins on uaTrans.fkUserID equals ul.user_login_id_pk into ulGroup

Answer 1

因此您的数据库至少包含两个 table：Users 和 UserDocuments。每个 UserDocument 都有一个属性 fkDocumentId。

虽然你没这么说，但在我看来，这是 table Users 中元素的外键。显然这个外键有时指的是 User.TransportUserId 有时指的是 User.WorkUserId.

你确定要这个吗？如果 fkDocumentId 的值为 10，它是指 TransportId 等于 10 的用户，还是指 WorkUserId 等于 10 的用户，还是两者？

无论如何，如果您调查 Enumerable.Join,，您会发现您提供了两个 keySelector：一个用于 select 来自 Users 的密钥，一个用于 select 来自 UserDocuments 的密钥。当这两个键相等时，ResultSelector 参数用于构造您的连接元素。

问题出在字"are equal"上。您必须确保 select 您的密钥并提供 IEqualityComparer 以便密钥将被视为相等。

另一个更简单的方法是创建一个新的 Join 扩展方法。

IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult> Join(
    this IEnumerable<TSource> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector1,
    Func<TOuter, TKey> outerKeySelector2,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, TInner, TResult> resultSelector)
{
    // make two lookup tables from Outer: one for key1 and one for key2:
    var lookup1 = outer.ToLookup( outerElement => outerKeySelector1(outerElement));
    var lookup2 = outer.ToLookup( outerElement => outerKeySelector2(outerElement));

    // so if you have a TKey from the innerKeySelector, you can find if it matches key1 or key2
    foreach (TInner innerElement in inner)
    {
        TKey innerKey = innerKeySelector(innerElement)
        var matchingElementsKey1 = lookup1[innerKey];
        var matchineElementsKey2 = lookup2[innerKey];
        var allmatchingElements = matchingElementsKey1.Concat(matchingElementsKey2);

        foreach(var matchingElement in allMatchingElements)
        {
            TResult result = resultSelector(matchingElement, innerElement);
            yield return result;
        }
    }
}

用法：

var result = tblUsers.Join(tblUserDocuments,
    user => user.TransportUserId,           // select outerKey1
    user => user.WorkUserId,                // select outerKey2
    document => document.fkDocumentId,      // select innerKey

    // when inner key matches either outerKey1, or outerKey2, create one new object:
    (user, document) => new
    {
        // Select the user documents that you want:
        UserId = user.Id,
        Name = user.Name,
        ...

        // Select the document properties that you want:
        DocumentId = document.Id,
        Author = document.Author,
        PublishedDate = document.Date,
        ...
    })

我如何将 "or" 运算符引入 linq 查询连接

How and I introduce an "or" operator into a linq query join

c#

sql

linq

entity-framework

linq-to-sql