can not retrieve data of navigation properties in EF 5, MVC 4

问题

I have the following PersonelInCourse entity:

public partial class PersonnelInCourse
{
    public int ID { get; set; }
    public string PersonnelID { get; set; }
    public string CourseID { get; set; }
    public int HoursInCourse { get; set; }
    public byte IsApproved { get; set; }
    public byte IsPassed { get; set; }
    public Nullable<int> RewardDetailID { get; set; }
    public string RejectReason { get; set; }
    public string FailReason { get; set; }

    public virtual Course Course { get; set; }
    public virtual RewardDetail RewardDetail { get; set; }
    public virtual Personnel Personnel { get; set; }
}

I insert data like this and it has no problem:

...
PersonnelInCourse p = new PersonnelInCourse();
p.CourseID = id;
p.PersonnelID = User.Identity.Name;
p.HoursInCourse = 0;
p.IsApproved = 0;
p.IsPassed = 0;            
db.PersonnelInCourses.Add(p);
try
{
    db.SaveChanges();
}
...

I try to retrieve this info somewhere like the following. However, this causes an exception and I found out that all of navigation properties are null, so the exception throws:

@{    
var psic = new Models.PtDbContext().PersonnelInCourses.Where(p => p.CourseID == Model.ID);
var c = new Models.PtDbContext().Courses.Find(Model.ID);
int i = 1;
} // these all are ok.
...

@foreach (var p in psic)
{
    <tr style="border-bottom: 1px dotted black;">
        <td>@i.ToString()</td>
        <td>@string.Concat(p.Personnel.FirstName, " ", p.Personnel.LastName)</td> 
    //the exception throws from here, because the navigation property Personnel is null, and all other navPrs also are null.
        <td>@p.Personnel.Post.PostName</td>
        <td>@p.PersonnelID</td>
    </tr>

    i++;
}

How can I achieve what I want? Where is my mistake?

回答1:

You should add Include() for explicit loading of navigation properties.

var psic = new Models.PtDbContext().PersonnelInCourses..Include("Personnel").Where(p => p.CourseID == Model.ID);

This will load PersonnelInCoursesand their Personnel in one query.

In case you need to load some more properties, simply chain more .Include("") clauses.

回答2:

Try keeping PtDbContext alive until you have finished processing data from it and explicitly test for the navigation property:

@using (var context = new Models.PtDbContext()){    
    var psic = context.PersonnelInCourses.Where(p => p.CourseID == Model.ID);
    var c = context.Courses.Find(Model.ID);
    int i = 1;
<table>
    @foreach (var p in psic)
    {
    <tr style="border-bottom: 1px dotted black;">
        <td>@i.ToString()</td>
        @if (p.Personnel == null)
        {
        <td><b>not found</b></td> 
        <td><b>not found</b></td>
        }
        else
        {
        <td>@string.Concat(p.Personnel.FirstName, " ", p.Personnel.LastName)</td> 
        <td>@p.Personnel.Post.PostName</td>
        }
        <td>@p.PersonnelID</td>
    </tr>
</table>
        @i++;
    }
}

回答3:

Found it! (based on comments of @Liel):

var psic = new jqPersonnelTraining.Models.PtDbContext().PersonnelInCourses
    .Include("Personnel").Include("Personnel.Post").Where(p => p.CourseID == Model.ID);

dear Liel, you must add .Include() before .Where() ... Thanks a lot for your tips.

来源：https://stackoverflow.com/questions/16829007/can-not-retrieve-data-of-navigation-properties-in-ef-5-mvc-4