Problema ao deletar um item da Tabela Principal, nao deletar da tabela secundaria, ter que inserir no context tanto item principal quanto secundario

     public class Dimension
    {
        public override string ToString()
        {
            return "X = " + this.X + ", Y = " + this.Y + ", Z = " + this.Z;
        }

            public string X { get; set; }
            public string Y { get; set; }
            public string Z { get; set; }
            public Dimension(string x = "", string y = "", string z = "")
            {

                this.X = x;
                this.Y = y;
                this.Z = z;
            }
        public Object3D objeto3d { get; set; }
    }


  public class Object3D 
    {

        public override string ToString()
        {
            return this.Description;
        }
        public int Id { get; set; }
        public string Name { get; set; }

        public string Drawing { get; set; }
        public string Description { get;  set; }


        public int IDParent
        { get; set; }

        public int IDChild
        { get; set; }



        public  Dimension dimension { get; set; }
}


       protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.CreateTable(
                name: "Dimensions",
                columns: table => new
                {
                    Object3dID = table.Column<int>(nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    X = table.Column<string>(nullable: true),
                    Y = table.Column<string>(nullable: true),
                    Z = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_Dimension", x => x.Object3dID);
                });
            migrationBuilder.CreateTable(
                name: "Object3D",
                columns: table => new
                {
                    Id = table.Column<int>(nullable: false),
                    Description = table.Column<string>(nullable: true),
                    Drawing = table.Column<string>(nullable: true),
                    IDChild = table.Column<int>(nullable: false),
                    IDParent = table.Column<int>(nullable: false),
                    Name = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_Object3D", x => x.Id);
                    table.ForeignKey(
                        name: "FK_Object3D_Dimension_Id",
                        column: x => x.Id,
                        principalTable: "Dimensions",
                        principalColumn: "Object3dID",
                        onDelete: ReferentialAction.Cascade);
                });
        }