LINQ query using TPH inheritance - get derived class ICollection count

Question

I am trying to figure out how to get the count of ProjectSummaryVotes on a specific ProjectDoc.OfType. The query works but the ProjectSummaryVote attribute is null, which means I get a null value error. Is this doable? Should I abandon TPH inheritance if I need to do this?

Model:

public class Project
{
    public int ProjectID { get; set; }
    public virtual ICollection<ProjectDoc> ProjectDoc { get; set; }

}
public abstract class ProjectDoc
{
    public int ProjectDocID { get; set; }
    public int ProjectID { get; set; }
    public string DocTitle { get; set; }
    public string Status { get; set; }
    public DateTime DateCreated { get; set; }

    public virtual Project Project { get; set; } 
    public virtual ICollection<Comment> Comment { get; set; }
}

public class Summary : ProjectDoc
{
    [DataType(DataType.MultilineText)]
    public string Text { get; set; }
    public ICollection<ProjectSummaryVote> ProjectSummaryVote { get; set; }
}
public class ProjectSummaryVote
{
    public int ProjectSummaryVoteID { get; set; }
    public int ProjectDocID { get; set; }
    public bool Vote { get; set; }
    public virtual Summary Summary { get; set; }
    public virtual User User { get; set; }
}

Query:

 Project project = db.Projects
          .Include(i => i.User)
          .Include(i => i.ProjectDoc)
          .SingleOrDefault(x => x.ProjectID == id);

Attempt to get the count that throws an error.

 var trueCount = Model.ProjectDoc.OfType<Literrater.Models.Summary>()
      .Where(s => s.Status == "Active")
      .SelectMany(v => v.ProjectSummaryVote.Where(s => s.Vote == true))
      .Count();

Answer 1

I remodeled the data so that all of the votes are now part of a single Vote model. That let me move the vote relationship out of the derived model and into the primary model, which then makes accessing the data easy.