以OrganizationUnit为例，进行扩展，加入IsUse属性
1.创建一个新类，比如ExtendedOrganizationUnit，继承OrganizationUnit
public class ExtendedOrganizationUnit: OrganizationUnit
{
public bool IsUse { get; set; }
}
2.把ExtendedOrganizationUnit添加到DbContext里
public virtual DbSet<ExtendedOrganizationUnit> ExtendedOrganizationUnits { get; set; }
3.使用ef code first迁移命令
add-migration add_ExtendedOrganizationUnit_table
update-database
成功