Question

Why does my model go out to the DB again for related properties which have already been loaded in a separate LINQ join statement?

Background info:

I recently ran some Performance profiling on my ASP.NET MVC3 Linq application I got some puzzling results that I hope to resolve.

Here is a simplified version of db structure. Foreign key relationships are set up.

Table Client:

ID|Client Name|Vertical ID|
___________________________
1 |Client1    |2          |
2 |Client2    |5          |
3 |Client3    |           |

Table Vertical

ID|Vertical Name|
_________________
2 |Life         |
5 |Guilt        |

When I display the list of clients, I am displaying them the client name and the vertical. My Controller is simple:

public ActionResult Index()
    {
        ...
    var model = repository.GetAllClients();
        return View(model);
    }

Get all clients runs the join on the vertical table (as well as a few others).

 public IEnumerable<client> GetAllClients()
    {
        OperationsMetricsDataContext db = new OperationsMetricsDataContext();
        var clients = from c in db.clients
                      join v in db.verticals on c.vertical equals v into tmp1
                      from v in tmp1.DefaultIfEmpty()
                      join sm in db.cel_sm_staffs on c.sm_id equals sm.cel_sm_id into tmp2
                      from sm in tmp2.DefaultIfEmpty()
                      join cel in db.cel_sm_staffs on c.cel_id equals cel.cel_sm_id into tmp3
                      from cel in tmp3.DefaultIfEmpty()
                      select c;
        return clients;
    }

In my View, I then attempt to display the vertical name I have bound using this method in my partial client class

public string getVerticalName()
    {
        return this.vertical == null ? "N/A" : this.vertical.vertical_name;
    }

The problem:

@foreach (var item in Model) {
<tr>
    <td>
        @Html.Raw(item.client_name)
    </td>
    <td>
        @Html.Raw(item.getVerticalName())
    </td>

The problem that I uncovered is that even though I specified the join in the original model, (and I’ve been able to verify that the vertical property has a value loaded before the model is passed to the view), each time I call the getVerticalName method, Linq automatically goes out to the db again to load the vertical so that I can access the vertical name. This is pretty inefficient because I end up with a separate db call for each client just to get their vertical information.

What am I missing here? Shouldn’t LINQ know that the property is already bound and not go out to the db to retrieve it or is there something I need to specify to tell LINQ that my information already exists in the model?

Answer 1

Not sure how it behaves in MVC, but perhaps your problem is lazy loading the related entity.
You can force Linq-2-sql to eagerly load using LoadOptions.

Try

OperationsMetricsDataContext db = new OperationsMetricsDataContext();

DataLoadOptions dlo = new DataLoadOptions();
dlo.LoadWith<Client>(c => c.Vertical);
db.LoadOptions = dlo;