如何在实体框架中一次显式加载多个模型的关系？

Question

I have a model similar to the following:

class Parent {
    public int Id { get; set; }
    public string Name { get;set; }
    public ICollection<Child> Children { get; set; }
    public GrandChildren SpecialGrandChild { 
        get {
            return Children.SelectMany(c => c.Children).Where(...).Single();
        }
    }
}

class Child {
    public int Id { get; set; }
    public int ParentId { get; set; }
    public Parent Parent { get; set; }
    public ICollection<GrandChild> Children { get; set; }
}

class GrandChild {
    public int Id { get; set; }
    public string Name { get;set; }
    public int ParentId { get; set; }
    public Child Parent { get; set; }
}

I also have a fairly complex query involving all three tables. From that query I want to extract all the Parent objects, and I will be displaying a property of the SpecialGrandChild for each one.

The problem is that if I do:

query.Include(p => p.Children.Select(c => c.Children));

EF will generate an ungodly sql query, and take a ton of time to build the query (on some cases over 10 seconds!). The query is cached so further calls are much faster. If I drop the Include call, I do not get such a bad first-call performance, but of course I get a worse performance as I will be doing M*N+1 queries (for each Parent, fetch the Children, and for each Child fetch the GrandChildren).

So the question is: can I explicitly load all the Children and GrandChildren for all the loaded Parents in a single call? If so, how can I do so?

I tried querying all the Childs for the currently loaded Parents as follows:

var ids = parents.Select(p => p.Id);
(from c in Childs where ids.Contains(c.ParentId) select c).Include("Children").Load();

But that call does not tell EF that all the associated Childs are loaded so it still goes to the DB when I access the association properties.

Answer 1

Load your data in two steps:

var dbParent = ...; // query all Parent's
var dbChild = ...; // query all Child's

var parents = dbParent.Include(p => p.Children).ToList();
dbChild.Include(p => p.Children).toList();

That should make parents have a list of all parents, and because of tracking, each parent will have each of its children.

If you have to apply a filter condition on parent, you should make it on children too.

Answer 2

Since I don't know enough about your parent variable and a context your parent objects are probably attached on (or maybe not) I can't give you a precise answer. But what you're looking for should look like this where ctx is an instance of your context and Parents a DbSet Property:

 IQueryable<Parent> query = ctx.Parents.Include("Children.Children");
 List<Parent> myTree = query.toList();