Linq选择最新的组层次结构

Question

I want to get the latest record for each group if records when self-referencing hierarchy exists.

My table looks like this:

RecordId CreatedDate ParentRecordId
   1     2012/05/13       NULL   
   2     2012/05/13       NULL   
   3     2012/05/14        1     
   4     2012/05/15        3     

I want to select only latest versions of records.
So in this case I only want to select RecordId = 2 and RecordId =4.

Here's what I have so far and I'm stuck.

     db.Records
       .Where(x => x.ParentRecordId!= null).GroupBy(x => x.ParentRecordId)
       .SelectMany(x=>x.OrderByDescending(y=>y.CreatedDate).Take(1)).ToList();

Answer 1

My left joins are a bit lacking, but something like this ought to do it;

var query = from r1 in db.Records
            join r2 in db.Records
                on r1.RecordId equals r2.ParentRecordId into rec
            from r in rec.DefaultIfEmpty()
            where r == null
            select r1;

Answer 2

How about a query that answers "Get all entries for which there is not another entry that considers me a parent". That sounds like the same thing, unless I misunderstand:

db.Records.Where(x => !db.Records
    .Select(r => r.ParentRecordId).Contains(x.RecordId))

However, I'm a little confused on what you mean by "circularity". How can a hierarchy be circular?

Answer 3

you should get a list of the ParentRecordId s first, and then check to see if the RecordId is in that list, if it is then we should exclude it from the results:

var parentIds = db.Records
                .Where(r => r.ParentRecordId != null)
                .Select(r => r.ParentRecordId)
                // this ToList call may increase performance. Depending 
                // on the size of the result set we may not want to call
                // the database every time for this info
                .ToList();
var wantedRecords = from r in db.Records
                    where !parentIds.Contains(r.RecordId)
                    select r;