title: Attempted Left Join in Query Syntax and Lambda results in "The method or operation is not implemented."
body: <p>I've attempted to write this query two different ways, and no matter how I write it, I can't get the join to pan out. It just keeps tossing <code>"The method or operation is not implemented."</code> at me. What I'm trying to do is, in SQL, simple: Obtain a list of items from table A (below - Customer) where there is no corresponding listing in table B (below - SalesReps).</p>
  
  <p>Attempted Lamda:</p>
  
  <pre><code>var customers =
      _sms.CurrentSession.Query&lt;customer&gt;()
          .GroupJoin(_sms.CurrentSession.Query&lt;salesReps&gt;(), 
                      c =&gt; c.Id, 
                      sr =&gt; sr.CustomerId,
                      (x, y) =&gt; new {c = x, sr = y})
          .SelectMany(xy =&gt; xy.sr.DefaultIfEmpty(), 
                      (x, y) =&gt; new {c = x.c, sr = y})
          .Where(data =&gt; data.sr.SalesRepId == null)
          .Select(data =&gt; new CustomerDTO
              {
                  Id = data.c.Id,
                  FullName = data.c.FirstName + " " + data.c.LastName,
                  FirstName = data.c.FirstName,
                  LastName = data.c.LastName
              });
  
  var custList = customers .ToList();
  </code></pre>
  
  <p>Attempted Query Syntax:</p>
  
  <pre><code>var customers = from c in _sms.CurrentSession.Query&lt;customer&gt;()
      join sr in _sms.CurrentSession.Query&lt;salesReps&gt;() 
          on c.Id equals sr.CustomerId 
              into joinedData
      from jd in joinedData.DefaultIfEmpty()
      where c.IsEmployee == false 
              &amp;&amp; c.CustomerOptions.Company.Id == companyGuid
              &amp;&amp; jd.SalesRepId == null
      select
          new CustomerDTO
          {
              Id = c.Id,
              FullName = c.FirstName + " " + c.LastName,
              FirstName = c.FirstName,
              LastName = c.LastName
          };
  
  var custList = customers .ToList();
  </code></pre>
  
  <p>I get the impression that the issue is on the check for null sales reps, but I'm not sure.</p>