c# - 我如何将 "or"运算符引入 linq 查询连接

Question

在 linq 查询连接中引入“or”运算符的语法是什么。好像不太喜欢“||”、“或”？我需要将一个外键与用户表中的两个可能的键连接起来。

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，这可以工作，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，您有一个至少包含两个表的数据库:Users 和 UserDocuments。每个UserDocument都有一个属性fkDocumentId。

虽然你没有这么说，但在我看来，这是表 Users 中元素的外键。显然，这个外键有时引用 User.TransportUserId，有时引用 User.WorkUserId。

你确定要这个吗？如果 fkDocumentId 的值为 10，那么它是指 TransportId 等于 10 的用户，还是 WorkUserId 等于 10 的用户，或者两者兼有？

无论如何，如果你调查Enumerable.Join,然后你会发现你提供了两个keySelector:一个从Users中选择一个key，一个从UserDocuments中选择一个key。当这两个键相等时，ResultSelector 参数将用于构造您的连接元素。

问题出在“平等”这两个字上。您必须确保选择 key 并提供 IEqualityComparer，以便 key 被视为相等。

另一种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,
        ...
    })