Linq-to-Sql:递归获取子级 | 递归获取子级

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问题描述

我有一个Comment表，其中包含CommentID和ParentCommentID.我正在尝试获取评论的所有子级的列表.到目前为止，这是我到目前为止尚未完成的测试.

I have a Comment table which has a CommentID and a ParentCommentID. I am trying to get a list of all children of the Comment. This is what I have so far, I haven't tested it yet.

private List<int> searchedCommentIDs = new List<int>();
// searchedCommentIDs is a list of already yielded comments stored
// so that malformed data does not result in an infinite loop.
public IEnumerable<Comment> GetReplies(int commentID) {
    var db = new DataClassesDataContext();
    var replies = db.Comments
        .Where(c => c.ParentCommentID == commentID
            && !searchedCommentIDs.Contains(commentID));
    foreach (Comment reply in replies) {
        searchedCommentIDs.Add(CommentID);
        yield return reply;
        // yield return GetReplies(reply.CommentID)); // type mis-match.
        foreach (Comment replyReply in GetReplies(reply.CommentID)) {
            yield return replyReply;
        }
    }
}

2个问题:

有什么明显的方法可以改善这一点吗? (此外，也许可以在SQL中使用CTE创建视图.)
为什么我不能只给Comment产生IEnumerable <Comment>到IEnumerable <Comment>?
在这种情况下是否仍然可以使用SelectMany?

Is there any obvious way to improve this? (Besides maybe creating a view in sql with a CTE.)
How come I can't yield a IEnumerable <Comment> to an IEnumerable <Comment>, only Comment itself?
Is there anyway to use SelectMany in this situation?

推荐答案

我可能会使用UDF/CTE，或者使用(对于非常深的结构而言)手动执行相同操作的存储过程.

I'd probably use either a UDF/CTE, or (for very deep structures) a stored procedure that does the same manually.

请注意，如果您可以更改架构，则可以将此类递归结构预索引到索引树/范围树中，从而可以执行单个BETWEEN查询-但树的维护成本很高(即查询变得便宜，但是插入/更新/删除变得昂贵，或者您需要延迟的计划任务.

Note that if you can change the schema, you can pre-index such recursive structures into an indexed/ranged tree that lets you do a single BETWEEN query - but the maintenance of the tree is expensive (i.e. query becomes cheap, but insert/update/delete become expensive, or you need a delayed scheduled task).

Re 2-您只能yield枚举中指定的类型(IEnumerable<T>/IEnumerator<T>中的T).

Re 2 - you can only yield the type specified in the enumeration (the T in IEnumerable<T> / IEnumerator<T>).

如果方法返回IEnumerable<IEnumerable<Comment>>，您可以yield一个IEnumerable<Comment> -这样做有意义吗?

You could yield an IEnumerable<Comment> if the method returned IEnumerable<IEnumerable<Comment>> - does that make sense?

改进:

也许是使用CTE递归方法的 udf (以保持可组合性，而不是存储过程)
使用using，因为DataContext是IDisposable ...

perhaps a udf (to keep composability, rather than a stored procedure) that uses the CTE recursion approach
use using, since DataContext is IDisposable...

如此:

using(var db = new MyDataContext() ) { /* existing code */ }

LoadWith 值得一试，但我不值得我一定会充满希望...
搜索到的ID列表作为一个字段是有风险的-只要您不两次调用它，我想您就可以了...就我个人而言，我会在私有支持方法上使用一个参数...(即在递归调用之间传递列表，但不在公共API上传递

LoadWith is worth a try, but I'm not sure I'd be hopeful...
the list of searched ids is risky as a field - I guess you're OK as long as you don't call it twice... personally, I'd use an argument on a private backing method... (i.e. pass the list between recursive calls, but not on the public API)

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