Using LINQ to query three entitites. - Include pat

问题:

I am trying to get data from 3 entities. Exam, Objective and Objective detail. I want to be able to select this by exam name. Here is the code I am using:

        var result = await db.Exams
            .Include(e => e.Objectives)
            .Include(e => e.Objectives.SelectMany(o => o.ObjectiveDetails))
            .Where(e => e.Name == name)
            .FirstOrDefaultAsync();

This is giving me an error message when I run it saying:

exceptionMessage=The Include path expression must refer to a navigation property defined on the type. Use dotted paths for reference navigation properties and the Select operator for collection navigation properties. Parameter name: path

Can someone advise me what I am doing wrong. Here's my classes:

public class Exam
{
    public Exam()
    {
        this.Objectives = new HashSet<Objective>();
    }
    public int ExamId { get; set; }
    public int SubjectId { get; set; }
    public virtual ICollection<Objective> Objectives { get; set; }
}

public class Objective : AuditableTable
{
    public Objective()
    {
        this.ObjectiveDetails = new HashSet<ObjectiveDetail>();
    }
    public int ObjectiveId { get; set; }
    public int ExamId { get; set; }
    public int Number { get; set; }
    public virtual Exam Exam { get; set; }
    public virtual ICollection<ObjectiveDetail> ObjectiveDetails { get; set; }

}

public partial class ObjectiveDetail
{
    public int ObjectiveDetailId { get; set; }
    public int ObjectiveId { get; set; }
    public int Number { get; set; }
    public string Text { get; set; }
    public virtual Objective Objective { get; set; }
}

回答1:

Use Select instead of SelectMany:

  .Include(e => e.Objectives.Select(o => o.ObjectiveDetails))

回答2:

Try this:

var result = await db.Exams
    .Include("Objectives.ObjectiveDetails")
    .Where(e => e.Name == name)
    .FirstOrDefaultAsync();

From http://msdn.microsoft.com/en-us/library/bb738708(v=vs.110).aspx

Query paths can be used with Entity SQL and LINQ queries. Paths are all-inclusive.

For example, if an include call indicates Include("Orders.OrderLines"), not only will OrderLines be included, but also Orders. For more information, see Shaping Query Results (Entity Framework).