EF 4.1 抽象集合加载问题

Question

I have a problem regarding EF 4.1 code first. I am trying to demonstrate with a simplified example.

Let's say we have a Country class which contains a States collection. State collection contains schools collection. School is an abstract class. It has specializations of ElementarySchool and HighSchool.

HighSchool has a collection property of DrivingCourses. DrivingCourses and all other data saves into the db successfully.

My problem is when I'm loading the Country class the DrivingCourses collection remains null. (everything else is ok)

As I understand the problem is because when ef loads and populates the HighSchool class it's not aware of the courses collection.

I am unable to add this mapping because with the fluent api's static reflecion I can only map properties of the (abstract) School class.

I'm using the default config for abstraction: Table per Hierarchy Could someone please brighten me, if it's possible to solve my problem with EF 4.1?

Thanks in advance, Sandor

Answer 1

If I understand your description correctly then your model looks roughly like this (I omit key properties and so on):

public class Country
{
    public ICollection<State> States { get; set; }
}

public class State
{
    public ICollection<School> Schools { get; set; }
}

public abstract class School { ...  }

public class ElementarySchool : School { ... }

public class HighSchool : School
{
    public ICollection<DrivingCourse> DrivingCourses { get; set; }
}

public class DrivingCourse { ... }

And you have a DbContext which includes public DbSet<Country> Countries { get; set; } public DbSet<Country> Countries { get; set; } public DbSet<Country> Countries { get; set; } .

Now you want to load all Countries (or a filtered collection of Countries) including all navigation properties (especially also the DrivingCourses ).

I don't know if this is possible with a single roundtrip to the database (by eager loading all collections). A solution which will require multiple roundtrips though might be this one:

// Load all Countries including `States` and `Schools` collection
// but not the `DrivingCourses` collection
var countryList = context.Countries
    .Include(c => c.States.Select(s => s.Schools))
    .ToList();

// Create in-memory list of all loaded Schools of type HighSchool
IEnumerable<HighSchool> highSchoolList =
    countryList.SelectMany(c =>
                  c.States.SelectMany(s => s.Schools.OfType<HighSchool>()));

// Explicitely load the DrivingCourses one by one
foreach (var item in highSchoolList)
    context.Entry(item).Collection(h => h.DrivingCourses).Load();

Just as a first idea. It's likely that there are better solutions.

Edit

Using Load on the Countries DbSet doesn't change the problem. Load is the same as ToList() without actually returning a result, the entities are just loaded into the context. The code above could be rewritten like so:

context.Countries.Include(c => c.States.Select(s => s.Schools)).Load();

IEnumerable<HighSchool> highSchoolList =
    context.Countries.Local.SelectMany(c =>
                  c.States.SelectMany(s => s.Schools.OfType<HighSchool>()));

foreach (var item in highSchoolList)
    context.Entry(item).Collection(h => h.DrivingCourses).Load();

But this is basically the same as before and it also doesn't solve the problem to load the DrivingCourses in the first Load statement in a single DB roundtrip.