Question

I am getting a MismatchedTreeNodeException throw with the simple query below using NHibernate 3.2.0.4000. I am assuming it is a bug and if so does anyone know of a work around?

var result = session.Query<File>()
    .OrderBy(x => x.Author)
    .GroupBy(file => file.Author)
    .Select(author => new FileAuthor(author.Key, author.Count()))
    .ToList();

Answer 1

I have played around with your example and query in this form works fine:

var result = session.Query<File>()
                    .GroupBy(file => file.Author)
                    .Select(author => new
                    {
                       Key = author.Key.AuthorId,
                       Count = author.Count()
                    })
                    .ToList();

Apparently, when you group by entity, it is possible only to project its ID and aggregations. It seems that sorting needs to be done on the client.

Used mappings:

 <class name="Author" table="authors">
        <id name="AuthorId" column="author_id" />

    <property name="AuthorName" column="author_name" />

    <bag name="Files">
      <key>
        <column name="author_id" />
      </key>
      <one-to-many class="File"/>
    </bag>

  </class>

  <class name="File" table="files">
        <id name="FileId" column="file_id" />

    <property name="FileName" column="file_name" />

    <many-to-one name="Author" class="Author">
      <column name="author_id"  />
    </many-to-one>

  </class>