I'm using EF Core 5.0 and I'm attempting to create a many-to-many relationship:

```
public class User
{
    public int Id { get; set; }
    public ICollection<UserItem> UserItems { get; set; }
}

public class Item
{
    public int Id { get; set; }
    public ICollection<UserItem> UserItems { get; set; }
}

public class UserItem
{
    public int Id { get; set; }

    public int UserId { get; set; }
    public User User { get; set; }

    public int ItemId { get; set; }
    public Item Item { get; set; }

    public int Quantity { get; set; }
}

```

As you can see, the `UserItem` class has an additional property called `Quantity`, I was wondering if this is the correct approach, and how would I map all this in the `DBContext` class?