EF Core no .Include() method on DBset - asp.net-core

Question

I'm currently completely unable to call .Include() and intellisense (in vscode) doesn't seem to think it exists.
Now after a long time searching the web I've found this:
Not finding .Include() method in my EF implementing Generic repository
which seems to suggest that .Include exists only in System.Data.Entities, which is only available for EF 5 and 6.
So how do i eager load my list property for an entity in EF core?
heres my context
public class Database : DbContext
{
//Set new datasources like this: public DbSet<class> name { get; set; }
public DbSet<Domain.Resource> Resources { get; set; }
public DbSet<Domain.ResourceType> ResourceTypes { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlite("Filename=./something.db");
}
}
Heres the data classes:
public class Resource
{
public int ResourceId { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public int ResourceTypeId { get; set; }
public ResourceType ResourceType { get; set; }
}
public class ResourceType
{
public int ResourceTypeId { get; set; }
public string Name { get; set; }
public List<Resource> Resources { get; set; }
}
Then I do something like:
public List<ResourceType> GetAll()
{
var router = new Database();
var result = router.ResourceTypes.Include(rt => rt.Resources); //It's here there's absolutely no .Include method
return result.ToList();
}
Does .Include not exist in EF Core?

Answer 1

It's a direct consequence of a missing reference in the file where I'm making a call to the method (though i'm not quite sure i understand how...)
Anyways, adding:
using Microsoft.EntityFrameworkCore;
like Tseng and Smit suggested, did the trick. (in the file in which i define the function)
Though why that works i have no idea. I thought .include would automatically be available through the DbSet.
Thanks though! :)
Small, late EDIT: as Christian Johansen pointed out in his comment, the reason it needs the import to see the method signature, is that it is an extension method, which is a topic I strongly encourage any up-and-coming C# developer to learn about as it is immensely useful.

Answer 2

If you end up here, a user of EF 6 or below and happen to miss that OP actually mentioned this like I did, you want to add
using System.Data.Entity;
to your class.

Answer 3

Here is a previous answer that is tracking this issue in EF7. It appears it is now 'included'.