public class ResourcePossibleUm : EntityBase
    {
        public virtual bool IsMain { get; set; }
        public virtual double UmMass { get; set; }
        -------
    }

public class ResourcePosibleUMMap : ClassMap<ResourcePossibleUm>
    {
        public ResourcePosibleUMMap()
        {
            Id(x => x.Id).GeneratedBy.Identity();
            Map(x => x.IsMain);
            Map(c => c.UmMass);
        }
    }