Eager Loading from a list of parent to children problem

I am querying to get a list of parent entities that have an eager load to a list of child enties per parent. Pretty standard stuff to combat N+1 and the I can see 2 select statements being fired in a single sql query, 1st to get the parents and 2nd to get the children.

The problem I am seeing is that each child collection in the parent is showing 0 items in the collection.

Getting 1 parent item by id works. The child collection is eager loaded correctly. Surely eager loading works against sets of parents?

I am attempting this using release 4.0.1688.21487

Thanks,

Jim

Yes - eager loading should be working consistently for a set of parents as it does for a single parent. Are you using a named aggregate at all to control eager loading?

Jeremy

Hi Jeremy, I have resolved it but I wonder if you can elaborate on why. I have a good idea but would like it confirmed.

The issue is down to how we pass a select lambda into a paging function. We have a linq query that has had the correct with aggregate statement applied to it. This query is then passed to a PagedList constructor as below:

var returnList = new PagedList<Question>(query.Select(x => x.AsResource()), criteria.Offset.GetValueOrDefault(1), criteria.Limit.GetValueOrDefault(250));

The paged list constructor is:

public PagedList(IQueryable<T> source, int offset, int limit)
{
  TotalCount = source.Count();

  Offset = offset;
  Limit = limit;
  AddRange(offset == 0 ? source.Take(limit) : source.Skip(offset).Take(limit));
}

Now the problem is here: Select(x => x.AsResource())

This for some reason doesn't allow the query to actually build an aggregated set of queries.

To fix it I created a AsPagedList extension method that calls a different PagedList constructor which accepts an IEnumerable and not IQueryable.:

public static PagedList<TOut> AsPagedList<TIn, TOut>(this IQueryable<TIn> entities, Func<TIn, TOut> asResource, int offset, int limit)
{
  var list = (offset == 0 ? entities.Take(limit) : entities.Skip(offset).Take(limit)).ToList().Select(asResource);
  return new PagedList<TOut>(list, entities.Count(), offset, limit, false);
}

The call to this changed to:

var returnList = query.AsPagedList(entity => entity.AsResource(), criteria.Offset.GetValueOrDefault(1), criteria.Limit.GetValueOrDefault(250));

And as the extension class executes the query on the ToList before the Select and asResource mapping occurs then the aggregate queries are issued.

Does this make sense?

Thanks,

Jim

If I am understanding you correctly the difference between those two queries is that the second one is using a projection via the .Select() call. When we perform a projection eager loads are ignored. In the first example because you are forcing an enumeration via .ToList() before the .Select() call we treat this as a standard entity query and eager loads are triggered. If you need the eager loads to be triggered you will need to structure your queries to perform enumeration prior to using a Select call.

Let me know if there is anything still fuzzy about how this works :)

Jeremy

Thanks Jeremy and yes this all makes sense.

ometimes we want to use eager loading as:-

1. The dataset returned is small
2. We don't care about the cartesain product.
3. We want to load the parents
4. We want to load the children
5. We want to filter the results based on a where clause on the children

So how do we go about this, the domain model for this example looks like this:- view plainprint? public class Movie : Entity
{
public virtual string Director { get; set; }
public virtual IList ActorList { get; set; }
}

public class Actor : Entity
{
public virtual string Name { get; set; }
public virtual string Role { get; set; }
}
When using NHibernate and QueryOver we would specify eager loading by using:- thomas

You will want to use the [EagerLoad] attribute on your backing field (or by toggling the Eager Load option for the entity property from the Designer). Have a read through http://www.mindscapehq.com/documentation/lightspeed/Performance-and-Tuning/Controlling-How-Entities-Load for a bit more detail on how to go about doing this and let us know if you have any other questions on this :)

Jeremy