Nhibernate: How to represent Many-To-Many relationships with One-to-Many relationships?

Question

I have read a post on the internet (I can no longer find that post for me to refeence) that a Many-To-Many relationship can be replaced with a one-to-many relationship. Can

Answer 1

I just come up to that question, and realized, that there is missing any answer. And it is a shame, while I do often point out this NHibernate documentation statement: 24. Best Practices

Don't use exotic association mappings.

Good usecases for a real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, we think that most associations are one-to-many and many-to-one, you should be careful when using any other association style and ask yourself if it is really neccessary.

Take a look at the example under the 23.2. Author/Work. Extract, the simplified version of the many-to-many relation between Author and Work:

        
        ...
        
            
                
            
            
                
            
        

And its many-to-many target Author:

  ...
  
     
     
  

So, if we would like to order the set of Works on load, we do have a problem. There is no column in the pair table. But what's more important, there is no way how to manage such a column.

What we can do, is to introduced the Pair object: AuthorWork and extend the Pair table as needed

public class AuthorWork
{

    public virtual Author Author { get; set; }
    public virtual Work Work { get; set; }
    public virtual int OrderBy { get; set; }
}

Mapping of the AuthorWork

    ...
    
    
    

Having this we can convert the many-to-many mapping to one-to-many, for example the Authors collection:

And we can manage the entity AuthorWork, set the OrderBy column, and therefore effectively work with the pairing table.

NOTE: have to agree with that suggestion in docsumentation The more requirements come, the more happy we are that we do have a way how to manage the relation!