Many-To-Many relationships with two natural keys - nhibernate

Question

I have the following mapping
UserProfile.hbm.xml
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="AngusBook.Domain"
namespace="AngusBook.Domain">
<class name="UserProfile">
<id name="UserId" type="int">
<generator class="identity" />
</id>
<natural-id mutable="false">
<property name="UserName" />
</natural-id>
<set name="Companies" table="Users_Companies">
<key column="UserId"/>
<many-to-many column="CompanyId" class="Company" />
</set>
</class>
</hibernate-mapping>
Company.hbm.xml
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="AngusBook.Domain" namespace="AngusBook.Domain" >
<class name="Company">
<id name="Id">
<generator class="hilo" />
</id>
<natural-id mutable="false">
<property name="CompanyName" />
</natural-id>
<set name="Users" table="Users_Companies">
<key column="CompanyId"/>
<many-to-many column="UserId" class="UserProfile" />
</set>
</class>
</hibernate-mapping>
Table design
With this mapping I have, I can have two identical rows in Users_Companies table (ie: two rows with the same pair of foreign keys that belong to the UserProfile and Company table). Using mappings, how do I cause NHibernate or SQL to throw an error/exception when an attempt is made to insert a pair of foreign keys into Users_Companies that already exist in the table? I would like each row in Users_Companies to be unique and not have repetitive data.

Answer 1

I found out the problem. I didn't implement the Equals() and GetHashCode() correctly.
I ended up refactored my entities to use the following base entity class and that fixed the issue. No error is thrown when a duplicated entity is added to the set but the set will simply not add the duplicated entity.
Entity.cs
public abstract class Entity
{
public virtual TId Id { get; protected set; }
protected virtual int Version { get; set; }
public override bool Equals(object obj)
{
return Equals(obj as Entity<TId>);
}
private static bool IsTransient(Entity<TId> obj)
{
return obj != null &&
Equals(obj.Id, default(TId));
}
private Type GetUnproxiedType()
{
return GetType();
}
public virtual bool Equals(Entity<TId> other)
{
if (other == null)
return false;
if (ReferenceEquals(this, other))
return true;
if (!IsTransient(this) &&
!IsTransient(other) &&
Equals(Id, other.Id))
{
var otherType = other.GetUnproxiedType();
var thisType = GetUnproxiedType();
return thisType.IsAssignableFrom(otherType) ||
otherType.IsAssignableFrom(thisType);
}
return false;
}
public override int GetHashCode()
{
if (Equals(Id, default(TId)))
return base.GetHashCode();
return Id.GetHashCode();
}
}
public abstract class Entity : Entity<Guid>
{
}
UserProfile.cs
public class UserProfile : Entity<int>
{
public UserProfile()
{
Companies = new HashedSet<Company>();
}
public virtual string UserName { set; get; }
public virtual ISet<Company> Companies { set; get; }
}
Company.cs
public class Company : Entity<int>
{
public Company()
{
Users = new HashedSet<UserProfile>();
}
public Company(string name) :this()
{
this.CompanyName = name;
}
public virtual string CompanyName { set; get; }
public virtual ISet<UserProfile> Users { set; get; }
}