Question

I’m trying to fetch first element from the collection like this

List<Entity> data = session.Query<Entity>()
                       .Fetch(x => x.Photos.First())
                       .ToList();

and I’m getting this error.
A fetch request must be a simple member access expression; ‘[100002]’ is a SubQueryExpression instead. Parameter name: relatedObjectSelector.

Now I’m using .Fetch(x => x.Photos.First()) cause I know that first element will always be populated, and it is. I do need just first element from the collection to reduce loading time, so this is exact solution I need, but I’m getting this error.

Answer 1

Fetch will fetch the entire collection, you can’t tell it to just fetch the first element using “fetch”, however you can probably get the desired effect using projections, or something like:

        var subQ = QueryOver.Of<Entity>()
            .SelectList(x => x.SelectMin(y => y.Photos)); // sub query selecting min photo id from entity

        List<Photo> data = session.QueryOver<Photo>()                
                               .Fetch(x => x.Entity).Eager // eager load the entity
                               .WithSubquery.WhereProperty(x => x.Id).In(subQ) // restrict the photo to those that are the first ones for the entity
                               .List() // execute query from database
                               .ToList(); // convert to List

Although I’m not sure why you’d want a List instead of IList.. generally it’s preferable to use the interface