title: Group By List<Item>
body: <p>I have this class: </p>
  
  <pre><code>public class Item 
  {
      public virtual int32 Id { get; set; }
      public virtual Previa Previa { get; set; }
      public virtual Product Product { get; set; }
      public virtual Int32 Quantity { get; set; }
      public virtual Decimal Price { get; set; }
      public virtual Decimal Total { get; set; }
  }
  </code></pre>
  
  <p>And now i need to a query to find every Items in database, group by Products, with the sum of Quantity and Total.
  For find every Items, i use:</p>
  
  <pre><code>public List&lt;Item&gt; FindItem(int IdProduct = 0)
  {
      var retorno = (from c in Session.Query&lt;Item&gt;()
                     select c);
  
      if (IdProduto &gt; 0)
          retorno = retorno.Where(x =&gt; x.Product.Id == IdProduct)
  
      return retorno.ToList&lt;Item&gt;();
  }
  </code></pre>
  
  <p>But I don't know how to group these items. Could someone please help me?</p>