Fluent Hibernate One-To-One mapping issue - c#-4.0

Question

I am not sure how to map below given entities.
Below are the tables.
Employee { ID, Name, Role_ID } (Role_ID is foreign key from Role table)
Role {Role_ID, Name }
Below are the classes:
class Employee
{
public virtual string ID { get; set; }
public virtual string Name { get; set; }
public virtual Role EmpRole { get; set; }
}
class Role
{
public virtual string RoleID { get; set; }
public virtual string Name { get; set; }
}
Below are the Mappings:
public class EmployeeMap : ClassMap<Employee>
{
public EmployeeMap()
{
Table("Employee");
Id(x => x.ID, "ID");
Map(x => x.Name, "NAME");
//for relationship not sure which mapping to be used???
}
}
public class RoleMap : ClassMap<Role>
{
public RoleMap()
{
Table("Role");
Id(x => x.RoleID, "ROLE_ID");
Map(x => x.RoleID, "ROLE_ID");
Map(x => x.Name, "ROLE_NAME");
//For relationship not sure what to be used????
}
}
Scenario : One Employee will have one role. One Role can be assigned to multiple employee.
Please suggest me how to write the relationship for both entities?

Answer 1

You are facing the most common scenario. The mapping in this case would be many-to-one which in fluent is represented with .References()
public EmployeeMap()
{
Table("Employee");
Id(x => x.ID, "ID");
Map(x => x.Name, "NAME");
References(x => x.EmpRole , "Role_ID");
}
The best overview (I am aware about) is available here:
Mapping-by-Code - ManyToOne by Adam Bar
Suprisingly this post is not about fluent NHibernate but about mapping by code. The second half however, compares with fluent - and provides really comprehensive overivew. Small snippet:
References(x => x.PropertyName, "column_name")
.Class<ClassName>()
.Cascade.SaveUpdate()
.Fetch.Join()
.Update()
.Insert()
.ReadOnly()
.Access.Field()
.Unique()
.OptimisticLock()
.LazyLoad(Laziness.Proxy)
.PropertyRef(x => x.PropertyRef)
.NotFound.Ignore()
.ForeignKey("column_fk")
.Formula("arbitrary SQL expression")
.Index("column_idx")
.Not.Nullable()
.UniqueKey("column_uniq");
In case you would search for NHibernate doc (fluent operates on top of it) you should check the:
5.1.10. many-to-one
If you want Fluent-NHibernate doc:
Fluent mapping
Finally, if you like to see which Employees belong to each Role, you can extend your POCO:
class Role
{
public virtual string RoleID { get; set; }
public virtual string Name { get; set; }
public virtual IList<Employee> Employees { get; set; }
}
And map it as a <bag>.
HasMany(x => x.Employees)
.KeyColumn("Role_ID")
.Inverse()
.BatchSize(25)
See:
6.1. Persistent Collections (NHibernate doc for one-to-many)
Mapping-by-Code - Set and Bag - again by Adam Bar, second half about fluent