Question

I’m using EF4. Having 2 entities:
Person { Name }
Hobbys { Person.Name, IsCoolHobby }
1 Person can have several hobbys.

I now have

IQueryable<Person> p;
p = container.PersonSet.Include("Hobbys").AsQueryable();
p = p.Where(x => x ?????);
List<Person> tmp = p.ToList();

How can i return only those Persons who have cool hobbys (IsCoolHobby == true)? I tried join but i was not able to load them into the list (select can only return Person, Hobby or new Type – but how to map them to entity objects again?)

Thanks

Answer 1

How can i return only those Persons who have cool hobbys (IsCoolHobby
== true)?

List<Person> tmp = container.PersonSet.Include("Hobbys")
    .Where(p => p.Hobbys.Any(h => h.IsCoolHobby))
    .ToList();

This will load the people who have at least one cool hobby but the Hobbys collection for those people will always contain all hobbys, also the uncool hobbys.

Edit

Unfortunately filtering and sorting children during eager loading (Include) is currently not supported. There is a request on the EF feature suggestion page for this feature. The request has status “Under review”, so there is a little hope that it might get implemented in the future. (Probably far future: At least the first docs about EF 5 (beta) on MSDN say explicitly that eager loading with filtering/sorting is still not implemented.)

For now there are only two workarounds. The first is to use a projection:

var projectedData = container.PersonSet
    .Where(p => p.Hobbys.Any(h => h.IsCoolHobby))
    .Select(p => new
    {
        Person = p,
        CoolHobbys = p.Hobbys.Where(h => h.IsCoolHobby)
    })
    .ToList();

The result is a collection of anonymous objects which contain a user who has cool hobbys and a collection of those cool hobbys. If you don’t disable change tracking (by using the NoTracking option for the query) the person’s hobbys collection should be filled with the result automatically.

The second option is to use “explicit” loading with CreateSourceQuery:

List<Person> tmp = container.PersonSet
    .Where(p => p.Hobbys.Any(h => h.IsCoolHobby))
    .ToList();
foreach (var person in tmp)
{
    person.Hobbys.Attach(person.Hobbys.CreateSourceQuery()
        .Where(h => h.IsCoolHobby).ToList());
}

Two things to note here:

• CreateSourceQuery is only available on EntityCollections, i.e. if you are using EntityObject derived entities. It’s not available for POCO entities in EF 4.0. (EF >= 4.1/DbContext has the option for explicit loading also for POCOs -> Query() method.)
• The above code represents 1+N roundtrips to the database: The first for the person collection without the hobbys and then one additional query per person to load the cool hobbys.