public class Permission : IEntity
{
        protected Permission()
        {
        }

        public Permission(string name, string mneminic)
        {
            Description = name;
            Mnemonic = mneminic;
        }

        public virtual int Id { get; set; }
        public virtual string Description { get; protected set; }
        public virtual string Mnemonic { get; protected set; }
 }

public class UserRole : IEntity
    {
        private readonly ISet<Permission> _permissions;

        protected UserRole() { }

        public UserRole(string name)
        {
            Name = name;
            _permissions = new HashSet<Permission>();
        }

        public virtual int Id { get; protected set; }

        public virtual string Name { get; protected set; }

        public virtual ReadOnlyCollection<Permission> Permissions
        {
            get { return new ReadOnlyCollection<Permission>(new List<Permission>(_permissions)); }
        }

        public virtual bool HasPermission(string mnemonic)
        {
            return _permissions.Any(x => x.Mnemonic.Equals(mnemonic, StringComparison.OrdinalIgnoreCase));
        }

        public virtual void AddPermission(Permission permission)
        {
            if (!_permissions.Contains(permission))
            {
                _permissions.Add(permission);
            }
        }

        public virtual void RemovePermission(Permission permission)
        {
            if (_permissions.Contains(permission))
            {
                _permissions.Remove(permission);
            }
        }
    }

public class PermissionMapping : ClassMap<Permission>
    {
        public PermissionMapping()
        {
            Id(x => x.Id);

            Map(x => x.Description);
            Map(x => x.Mnemonic);
        }
    }

public class UserRoleMapping : ClassMap<UserRole>
    {
        public UserRoleMapping()
        {
            Id(x => x.Id);
            Map(x => x.Name);

            HasManyToMany(x => x.Permissions)
                .AsSet()
                .Access.CamelCaseField(Prefix.Underscore)
                .Cascade.SaveUpdate()
                .ParentKeyColumn("RoleId")
                .ChildKeyColumn("PermissionId")
                .Table("[Roles_Permissions]");
        }
    }

var p1 = new Permission("Permission1", PermissionMnemonics.CanEditClaims);
var p2 = new Permission("Permission2", PermissionMnemonics.CanEditRoles);
var p3 = new Permission("Permission3", PermissionMnemonics.ClaimDontRequireAccept);
var p4 = new Permission("Permission4", PermissionMnemonics.CanAcceptClaim);

Session.SaveOrUpdate(p1);
Session.SaveOrUpdate(p2);
Session.SaveOrUpdate(p3);
Session.SaveOrUpdate(p4);

var role = new UserRole("Администратор");
role.AddPermission(p1);
role.AddPermission(p3);

Session.SaveOrUpdate(role);

NHibernate: INSERT INTO [Permission] (Description, Mnemonic) VALUES (@p0, @p1); select SCOPE_IDENTITY();@p0 = 'Permission1' [Type: String ( 4000 )], @p1 = 'EditClaims' [Type: String ( 4000 )]
NHibernate: INSERT INTO [Permission] (Description, Mnemonic) VALUES (@p0, @p1); select SCOPE_IDENTITY();@p0 = 'Permission2' [Type: String ( 4000 )], @p1 = 'EditRoles' [Type: String ( 4000 )]
NHibernate: INSERT INTO [Permission] (Description, Mnemonic) VALUES (@p0, @p1); select SCOPE_IDENTITY();@p0 = 'Permission3' [Type: String ( 4000 )], @p1 = 'ClaimDontRequireAccept' [Type: String ( 4000 )]
NHibernate: INSERT INTO [Permission] (Description, Mnemonic) VALUES (@p0, @p1); select SCOPE_IDENTITY();@p0 = 'Permission4' [Type: String ( 4000 )], @p1 = 'AcceptClaim' [Type: String ( 4000 )]
NHibernate: INSERT INTO [UserRole] (Name) VALUES (@p0); select SCOPE_IDENTITY();@p0 = 'Администратор' [Type: String ( 4000 )]