在具有内部联接的LINQ查询中选择最新日期

Question

I have a LINQ query that queries two tables, and returns records successfully. I want it to only return the most recent record, but I'm having trouble getting it to work. It returns all of the records without an error.

My query is:

  var viewModel = from area in db.area
                  join sample in db.sample on area.location_id equals sample.location_id
                  from s in db.sample.OrderByDescending(s => s.sample_date).Take(1)

                  select new AreaViewModel { h_area = area, s_sample = sample };

                  return View(viewModel);

Totally stuck, help greatly appreciated!

Answer 1

You are calling Take() method at wrong place, you have to write something like:

var viewModel = (from area in db.area
                 join sample in db.sample on area.location_id equals sample.location_id
                 from s in db.sample
                 select new AreaViewModel 
                       { 
                         h_area = area, 
                         s_sample = sample 
                       }).OrderByDescending(s => s.s_sample.sample_date)
                         .FirstOrDefault();

if you want to return collection, then you can use Take(1) there instead of FirstOrDefault() , as it will not return collection.

Answer 2

I think what you are trying to achieve is this:

var area= (from area in db.area
           join sample in db.sample on area.location_id equals sample.location_id
           orderby sample.sample_date descending
           select new AreaViewModel { h_area = area, s_sample = sample }).FirstOrDefault();

Calling FirstOrDefault method you will get the most recent record due to it adds TOP(1) to the SQL query that is generated by your Linq query.