Question

Is it possible using DbContext / DbSet / DbQuery to conditionally eagerly load a navigation property. The query below will return parties that have a particular role.

What I need is to additionally only load the matching roles.

first attempt

var peopleWithRole = (from p in Party
  from r in p.Roles
  where r.RoleTypeId == 1
  select p).Include(_ => _.Roles);

This loads all the roles, and it’s obvious why once you look at it.

I have tried a few things. If I have to cast and create an objectquery to do it that would be fine, I just can’t figure out how.

second attempt

var objectContext = ((IObjectContextAdapter)this).ObjectContext;
var set = objectContext.CreateObjectSet<Party>();
var result = (from p in set.Where("it.Roles.RoleTypeId == 1")
              select p)
;
result.Dump();

The iterator cannot be used like this?

Any ideas?

Answer 1

Filtering and sorting with eager loading is not supported. The only way to load the data in a single roundtrip is a projection:

var peopleWithRole = (from p in Party
                      select new
                      {
                          Party = p,
                          Roles = p.Roles.Where(r => r.RoleTypeId == 1)
                      })
                      .ToList()
                      .Select(a => a.Party);

If the relationship is one-to-many EF (“relationship fixup”) will populate the filtered Roles collection automatically in each Party (if you don’t disable change tracking). If the relationship is many-to-many it doesn’t work that easy and you need to fill the collections manually after you have returned the result list of anonymous objects.

Last option is to vote for the eager loading filtering feature here: http://data.uservoice.com/forums/72025-entity-framework-feature-suggestions/suggestions/1015345-allow-filtering-for-include-extension-method … and hope it will be implemented in future EF releases.