I can't find "Include" method with lambda expression in Entity framework?

Question

I am using entity framework and I can't find include method like in this example:

using(ArticleExtractorEntities db=new ArticleExtractorEntities())  
{
    Preference pref= db.Preferences.Include(  

here i find only the function include with the parameter (string path) and I don't find any other overload so how can I use Include with lambda expression?

Answer 1

it's not in System.Linq. Add

using System.Data.Entity

Answer 2

Update. For those looking how it extend your linq query with .Include()

No longer is it:

using System.Data.Entity;

With netcoreapp1.0 it is:

using Microsoft.EntityFrameworkCore;