I have two classes (just recreating the problem):

public class User
{
    public virtual int Id { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }
    public virtual IList<OrgUnitMembership> OrgUnitMemberships { get; set; }
}

public class OrgUnitMembership
{
    public virtual int UserId { get; set; }
    public virtual int OrgUnitId { get; set; }
    public virtual DateTime JoinDate { get; set; }
    public virtual DateTime LeaveDate { get; set; }
}

There's a Fluent NHibernate map for both, of course:

    public class UserMapping : ClassMap<User>
    {
        public UserMapping()
        {
            Table("Users");

            Id(e => e.Id).GeneratedBy.Identity();
            Map(e => e.FirstName);
            Map(e => e.LastName);

            HasMany(x => x.OrgUnitMemberships)
.KeyColumn(TypeReflector<OrgUnitMembership>
.GetPropertyName(p => p.UserId))).ReadOnly().Inverse();
        }
    }
    public class OrgUnitMembershipMapping : ClassMap<OrgUnitMembership>
    {
        public OrgUnitMembershipMapping()
        {
            Table("OrgUnitMembership");

            CompositeId()
                .KeyProperty(x=>x.UserId)
                .KeyProperty(x=>x.OrgUnitId);

            Map(x => x.JoinDate);
            Map(x => x.LeaveDate);

            References(oum => oum.OrgUnit)
.Column(TypeReflector<OrgUnitMembership>
.GetPropertyName(oum => oum.OrgUnitId)).ReadOnly();
            References(oum => oum.User)
.Column(TypeReflector<OrgUnitMembership>
.GetPropertyName(oum => oum.UserId)).ReadOnly();
        }
    }

What I want to do is to retrieve some users based on criteria, but I would like to combine all columns from the Users table with some columns from the OrgUnitMemberships table, analogous to a SQL query:

select u.*, m.JoinDate, m.LeaveDate
from Users u inner join OrgUnitMemberships m on u.Id = m.UserId
where m.OrgUnitId = :ouid

I am totally lost, I tried many different options. Using a plain SQL query almost works, but because there are some nullable enums in the User class AliasToBean fails to transform, otherwise wrapping a SQL query would work like this:

return
    Session
        .CreateSQLQuery(sql)
        .SetParameter("ouid", orgUnitId)
        .SetResultTransformer(Transformers.AliasToBean<UserDTO>())
        .List<UserDTO>()

I tried the code below as a test (a few different variants), but I'm not sure what I'm doing. It works partially, I get instances of UserDTO back, the properties coming from OrgUnitMembership (dates) are filled, but all properties from User are null:

User user = null;
                OrgUnitMembership membership = null;
                UserDTO dto = null;
                var users = Session.QueryOver(() => user)
                    .SelectList(list => list
                        .Select(() => user.Id)
                        .Select(() => user.FirstName)
                        .Select(() => user.LastName))
                    .JoinAlias(u => u.OrgUnitMemberships, () => membership)
                    //.JoinQueryOver<OrgUnitMembership>(u => u.OrgUnitMemberships)
                    .SelectList(list => list
                        .Select(() => membership.JoinDate).WithAlias(() => dto.JoinDate)
                        .Select(() => membership.LeaveDate).WithAlias(() => dto.LeaveDate))
                    .TransformUsing(Transformers.AliasToBean<UserDTO>())
                    .List<UserDTO>();