title: String concatenation in GroupBy
body: <p>This query below doesn't work because <code>String.Join</code> is not translatable. </p>
  
  <p>PostgreSQL has the <a href="https://stackoverflow.com/a/43944/221683">string_agg(expression, delimiter)</a> feature though. </p>
  
  <p>Is there anyway to use it from Linq?</p>
  
  <pre><code>var vwTourWithCategorieses = Context.Tours
                  .Join(Context.TourCategories, t =&gt; t.TourId, tc =&gt; tc.TourId,
                      (t, tc) =&gt; new { t.TourId, t.Name, tc.CategoryId})
                  .Join(Context.Categories, x =&gt; x.CategoryId, c =&gt; c.CategoryId,
                      (x, c) =&gt; new { x.TourId, TourName = x.Name, CategoryName = c.Name})
                  .GroupBy(x =&gt; new { x.TourId, x.TourName },
                      (key, c) =&gt; new VwTourWithCategories
                      {
                          TourId = key.TourId,
                          Name = key.TourName,
                          Categories = string.Join(",", c.Select(i =&gt; i.CategoryName))
                      })
                  .ToList();
  </code></pre>