How do I delete related objects in Entity framework code first database?
===

DBContext class is

    public class VGDB : DbContext
        {        
            public DbSet<Planet> Planets { get; set; }
        }

And model looks like:

    public class Planet
        {
            [Key]
            public int Id { get; set; }
            public string Name { get; set; }
            
            ...
            
            public List<Building> Constructions { get; set; } 
        }


    public class Building
        {
            [Key]
            public int Id { get; set; }
            public decimal Lvl { get; set; }
            public string Type { get; set; }
        }

Repository class:

    public class VGDBRepository
        {
            private readonly VGDB _vgdb;
            ...
            public void RemovePlanets()
            {
                foreach (Planet planet in _vgdb.Planets)
                {
                    _vgdb.Planets.Remove(planet);
                }
                _vgdb.SaveChanges();
            }
            ...
        }

Entity Framework creates database with two tables: `Planets` and `Buildings`, related by `Planet_Id` field. When I call `RemovePlanets()` method of my `VGDBRepository` class it removes planets record from Planets table and set's `Planet_Id` field of all buildings, related with deleted planets, in Buildings table to null but not deletes them, so I have redundant records in database. I use `code-first` strategy to create database. How can I force Entity Framework to remove such type of related data???
