实体框架按日期选择每个组之一

Question

I have a table like this (Table name: Posts):

+----+--------------------------+-------+------------+
| id |         content          | type  |    date    |
+----+--------------------------+-------+------------+
|  0 | Some text                | TypeA | 2013-04-01 |
|  1 | Some older text          | TypeA | 2012-03-01 |
|  2 | Some even older texttext | TypeA | 2011-01-01 |
|  3 | A dog                    | TypeB | 2013-04-01 |
|  4 | And older dog            | TypeB | 2012-03-01 |
|  5 | An even older dog        | TypeB | 2011-01-01 |
+----+--------------------------+-------+------------+

Using a LINQ expression I want to find the newest content of each type, so the result should be

Some text | TypeA 
A dog     | TypeB

I have tried a few things but no point in pointing out non-working expressions.

Answer 1

If you want to get the whole Posts. You can try this:

var query = Posts.GroupBy(p => p.Type)
                  .Select(g => g.OrderByDescending(p => p.Date)
                                .FirstOrDefault()
                   )

Answer 2

I suppose you can group your Posts rows by type and then select first content from descending ordered by date collection of that type

from row in Posts 
group row by row.type 
into g
select new
{       
    Content  = (from row2 in g orderby row2.date descending select row2.content).FirstOrDefault(),
    Type = g.Key
}

Answer 3

Or using temporary result and predicate

var tmp = posts.GroupBy(x => x.type).Select(x => new {x.Key, date = x.Max(g => g.date)).ToArray();

var filter = PredicateBuilder.False<Post>();

foreach (var item in tmp)
{
    filter = filter.Or(x => x.type == item.Key && x.date == item.date);
}

var newestPosts = posts.Where(filter);

Answer 4

From memory, something like this should do it

var data = context.Posts.GroupBy(p => p.Type)
                        .Select(g => new { 
                                          Type = g.Key, 
                                          Date = g.OrderByDescending(p => p.Date)
                                                  .FirstOrDefault()
                                         }
               

This would give you a new anonymous type, but you can always map it to a class