R – NHibernate self-join many-to-many symmetric relationship (person friends problem)

nhibernatenhibernate-mapping

Is there any way to set-up a symmetric self-join relationship mapping in NHibernate? Suppose we have two tables:

Users
  id

Relations
  id
  user1
  user2
  relation_type

The User and Relation classes should look like this:

class User
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual ISet<Relation> Relations { get; set; }
}

class Relation
{
    public virtual int Id { get; set; }
    public virtual User User1 { get; set; }
    public virtual User User2 { get; set; }
    // Let's leave the RealationType as string for the sake of brevity 
    public virtual string RelationType { get; set; }
}

I do NOT want the relations table to have two rows for the same relation. But the relation MUST be symmetric, which means if there's a relation between two users, A and B, the Relations collection of the user A must contain a relation with user B and the relations of user B must contain a relation to A.

It sounds almost like a challenge. But, can someone solve this? Please, if you can, post the xml mapping. I'm not using Fluent.

Best Answer

You can use Key-Many-To-One mapping and remove the Id field from the relation entity. Also you better use inheritance for different relation types.

Related Solutions

.net – NHibernate not persisting many-to-many relationship

@efdee

I was having the same problem and spent almost two days on this. I had a many to many relationship and the link table wasn't being updated either. I'm new to NHibernate, just trying to learn it so take everything I say with a grain of salt.

Well it turned out that it's not Fluent NHibernate, nor the mapping, but me not understanding how NHibernate works with many-to-many. In a many-to-many relationship if collections on both entities aren't populated, NHibernate doesn't persist data to the link table.

Let's say I have this entities in a many-to-many relationship :


partial class Contact
{
   public string ContactName {get; set;}
   public IList Locations {get; set;}

}

partial class Location
{
   public string LocationName {get; set;}
   public string LocationAddress {get;set;}
   public IList Contacts {get;set;}
}

when I add to a location to Contact.Locations, I have to make sure that the contact is also present inside location.Contacts.

so to add a location i have this method inside my Contact class.


public void AddLocation(Location location)
        {
            if (!location.Contacts.Contains(this))
            {
                location.Contacts.Add(this);
            }
            Locations.Add(location);
        }

This seems to have solved my problem, but like I said I'm just picking up NHibernate and learning it, may be there's a better way. If anyone has a better solution, please post.

This is the post that pointed me to check both collections: http://www.coderanch.com/t/217138/Object-Relational-Mapping/link-table-of-ManyToMany-annotation

R – Fluent NHibernate question

What you're looking for is a of a set of elements, which in standard hbm mapping is:

<set name="Roles" table="UserRoles">
  <key column="UserID" />
  <element column="Role" />
</set>

For Fluent NHibernate you can map this like so:

HasMany<string>(x => x.Roles)
  .AsElement("Role");

You may need to also specify the key name using WithKeyColumn(string).

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