Question

I have 2 related Linq to SQL questions. Please see the image below to see what my Model looks like.

Question 1

I am trying to figure how to eager load the User.AddedByUser field on my User class/table. This field is generated from the relationship on the User.AddedByUserId field. The table is self-referencing, and I am trying to figure out how to get Linq to SQL to load up the User.AddedByUser property eagerly, i.e. whenever any User entity is loaded/fetched, it must also fetch the User.AddedByUser and User.ChangedByUser. However, I understand that this could become a recursive problem…

Update 1.1:

I’ve tried to use the DataLoadOptions as follows:

var options = new DataLoadOptions();
options.LoadWith<User>(u => u.ChangedByUser);
options.LoadWith<User>(u => u.AddedByUser);

db = new ModelDataContext(connectionString);
db.LoadOptions = options;

But this doesn’t work, I get the following exception on Line 2:

System.InvalidOperationException occurred
  Message="Cycles not allowed in LoadOptions LoadWith type graph."
  Source="System.Data.Linq"
  StackTrace:
       at System.Data.Linq.DataLoadOptions.ValidateTypeGraphAcyclic()
       at System.Data.Linq.DataLoadOptions.Preload(MemberInfo association)
       at System.Data.Linq.DataLoadOptions.LoadWith[T](Expression`1 expression)
       at i3t.KpCosting.Service.Library.Repositories.UserRepository..ctor(String connectionString) in C:\Development\KP Costing\Trunk\Code\i3t.KpCosting.Service.Library\Repositories\UserRepository.cs:line 15
  InnerException:

The exception is quite self-explanatory – the object graph isn’t allowed to be Cyclic.
Also, assuming Line 2 didn’t throw an exception, I’m pretty sure Line 3 would, since they are duplicate keys.

Update 1.2:

The following doesn’t work either (not used in conjuction with Update 1.1 above):

var query = from u in db.Users
            select new User()
            {
                Id = u.Id,
                // other fields removed for brevityy
                AddedByUser = u.AddedByUser,
                ChangedByUser = u.ChangedByUser,

            };
return query.ToList();

It throws the following, self-explanatory exception:

System.NotSupportedException occurred
Message="Explicit construction of entity type 'i3t.KpCosting.Shared.Model.User' in query is not allowed."

I am now REALLY at a loss on how to solve this. Please help!

Question 2

On every other table in my DB, and hence Linq to SQL model, I have two fields, Entity.ChangedByUser (linked to Entity.ChangedByUserId foreign key/relationship) and Entity.AddedByUser (linked to Entity.AddedByUserId foreign key/relationship)

How do I get Linq to SQL to eageryly load these fields for me? Do I need to do a simple join on my queries?, or is there some other way?

Linq to SQL eager loading on self referencing table http://img245.imageshack.us/img245/5631/linqtosql.jpg

Answer 1

Maybe you could try taking a step back and seeing what you want to do with the relation? I’m assuming you want to display this information to the user in e.g. “modified by Iain Galloway 8 hours ago”.

Could something like the following work? :-

var users = from u in db.Users
            select new
            {
              /* other stuff... */
              AddedTimestamp = u.AddedTimestamp,
              AddedDescription = u.AddedByUser.FullName,
              ChangedTimestamp = u.ChangedTimestamp,
              ChangedDescription = u.ChangedByUser.FullName
            };

I’ve used an anonymous type there for (imo) clarity. You could add those properties to your User type if you preferred.

As for your second question, your normal LoadWith(x => x.AddedByUser) etc. should work just fine – although I tend to prefer storing the description string directly in the database – you’ve got a trade-off between your description updating when ChangedByUser.FullName changes and having to do something complicated and possibly counterintuitive if the ChangedByUser gets deleted (e.g. ON DELETE CASCADE, or dealing with a null ChangedByUser in your code).