ADO.NET Entity Framework Hierarchical Data - wcf

Question

Consider following database model:
And following query code:
using (var context = new DatabaseEntities())
{
return context.Feed.ToHierarchy(f => f.Id_Parent, null);
}
Where ToHierarchy is an extension to ObjectSet<TEntity> as:
public static List<TEntity> ToHierarchy<TEntity, TProperty>(this ObjectSet<TEntity> set, Func<TEntity, TProperty> parentIdProperty, TProperty idRoot) where TEntity : class
{
return set.ToList().Where(f => parentIdProperty(f).Equals(idRoot)).ToList();
}
This would result in example JSON formatted response:
[
{
"Description":"...",
"Details":[ ],
"Id":1,
"Id_Parent":null,
"Title":"...",
"URL":"..."
},
{
"Description":"...",
"Details":[
{
"Description":"...",
"Details":[ ],
"Id":4,
"Id_Parent":3,
"Title":"...",
"URL":"..."
},
{
"Description":"...",
"Details":[
{
"Description":"...",
"Details":[
{
"Description":"...",
"Details":[ ],
"Id":7,
"Id_Parent":6,
"Title":"...",
"URL":"..."
}
],
"Id":6,
"Id_Parent":5,
"Title":"...",
"URL":"..."
}
],
"Id":5,
"Id_Parent":3,
"Title":"...",
"URL":null
}
],
"Id":3,
"Id_Parent":null,
"Title":"...",
"URL":null
}
]
As you may have noticed ToHierarchy method is supposed to (and apparently do indeed) retrieve all rows from a given set (flat) and return hierarchical representation of these as per "parent property".
When I was in the middle of my implementation I quick tried my code and surprisingly it worked! Now, I imagine how weird does this sound to many of you, but I really don't understand why or how that piece of code works, even though I kinda wrote it down on my own...
Could you explain how does it work?
P.S.: if you look closer, ToHierarchy is not near the same as .Include("Details").

Answer 1

It works because set.ToList will load all records from the database table to your application and the rest is done be EF and its change tracking mechanism which should ensure correct referencing between related entities.
Btw. you are filtering records in the memory of your application, not in the database. For example if your table contains 10000 records and your filter should return only 10, you will still load all 10000 from the database.
You will find that implementing this with EF is quite hard because EF has no support for hierarchical data. You will always end with bad solution. The only good solution is using stored procedure and some support for hierarchical queries in the database - for example common table expressions (CTE) in SQL server.
I just made this very simple example and it works as I described in comment:
public class SelfReferencing
{
public int Id { get; set; }
public string Name { get; set; }
public SelfReferencing Parent { get; set; }
public ICollection<SelfReferencing> Children { get; set; }
}
public class Context : DbContext
{
public DbSet<SelfReferencing> SelfReferencings { get; set; }
}
public class Program
{
static void Main(string[] args)
{
using (var context = new Context())
{
context.Database.Delete();
context.Database.CreateIfNotExists();
context.SelfReferencings.Add(
new SelfReferencing()
{
Name = "A",
Children = new List<SelfReferencing>()
{
new SelfReferencing()
{
Name = "AA",
Children = new List<SelfReferencing>()
{
new SelfReferencing()
{
Name = "AAA"
}
}
},
new SelfReferencing()
{
Name = "AB",
Children = new List<SelfReferencing>()
{
new SelfReferencing()
{
Name = "ABA"
}
}
}
}
});
context.SaveChanges();
}
using (var context = new Context())
{
context.Configuration.LazyLoadingEnabled = false;
context.Configuration.ProxyCreationEnabled = false;
var data = context.SelfReferencings.ToList();
}
}
}
It uses code first approach but internally it is same as when using EDMX. When I get data I have 5 entities in list and all have correctly configured Parent and Children navigation properties.