title: How to perform a group join in .NET Core 3.0 Entity Framework?
body: <p>With the changes to .NET Core 3.0 I am getting </p>
  
  <blockquote>
    <p>... NavigationExpandingExpressionVisitor' failed. This may indicate
    either a bug or a limitation in EF Core. See
    <a href="https://go.microsoft.com/fwlink/?linkid=2101433" rel="noreferrer">https://go.microsoft.com/fwlink/?linkid=2101433</a> for more detailed
    information.)  ---> System.InvalidOperationException: Processing of
    the LINQ expression 'GroupJoin, ...</p>
  </blockquote>
  
  <p>This is a really simple query so there must be a way to perform it in .NET CORE 3.0:</p>
  
  <pre><code> var queryResults1 = await patients
              .GroupJoin(
                  _context.Studies,
                  p =&gt; p.Id,
                  s =&gt; s.Patient.Id,
                  (p, studies) =&gt; new 
                  {
                      p.DateOfBirth,
                      p.Id,
                      p.Name,
                      p.Sex,
                     Studies =studies.Select(s1=&gt;s1)
                  }
              )
              .AsNoTracking().ToListAsync();
  </code></pre>
  
  <p>I am basically looking for a Linq query (or method syntax as above) which will join Studies onto Patients, and set Studies to an empty list or null if there are no studies for the given patient.</p>
  
  <p>Any ideas?  This was working in .NET Core 2.2.  Also the MSFT link above mentions that the key breaking change is related to client side evaluation and avoiding that the generated query reads entire tables which must then be joined or filtered client side.  However with this simple query, the join should be easily doable server side.</p>